(This post contains the internet tradition known as "spoilers").
So after three years of wandering space, fighting robots, and using the word "frak" in socially unacceptable situations, the rag-tag fugitive fleet finally manages to pull up their hand-brakes over a shining planet named Earth (anyone looking for jokes about flying motorbikes can go elsewhere, I am trying to not be too obvious today).
My previous musings regarding the show have all focused upon the nature of the Final Five and how their existence can be crow-barred into the established history of the show. Well, half a season later and we're almost no closer to any kind of answer than we were to begin with. Aside from Tigh's conversation with Adama, which may be more important than it appeared (I think it finally answered the question of whether Tigh was ever "replaced"; since if this was the case the dialogue would have accentuated how Tigh's revelation made no sense, rather than quickly glossing over it), the only real piece of information to puzzle over is D'Anna's comment that only 80% of the Final Five were aboard the Colonial Fleet.
There are four theories that spring from this comment. Firstly, D'Anna might have been lying; the only reason I can think of for that is the realisation that if everything went tits-up and the Colonials flushed the four, she would still have some chance of rescuing the last one. This seems pretty unlikely, though, since she presumably couldn't know it was exactly four of the five that have been "activated", and because she was prepared to blast the fleet out of space once Tigh, Anders and Tyrol were stuck in the airlock.
The second is that the fifth Cylon was aboard the base-star at the time. This has the advantage of meaning said Cylon might still be someone we would recognise when they show up, but if this was the case, why wouldn't D'Anna talk to them in person, especially once Tory arrived and thus gave D'Anna some indication that the Final Five had been awoken.
Option three is a theory Senior Spielbergo kicked around a while back, that the final Cylon is already dead, which has some interesting possibilities (although the main one that Sen. S suggested, namely that the final Cylon might be Adama's father, doesn't really make sense in terms of the time line, even by the standards of recent revelations), but I think after knowing for the last five years that there are twelve models, I'm not sure the writers would be likely to toss away one of their final trump cards like that. They might be able to do something pretty good with it, but a priori it's hard to see how it wouldn't seem a bit of a let-down.
Of course, the fourth and final option is that the final Cylon is still at Earth, having stayed there when the other four returned to the Colonies. Why they would choose to do that is, obviously, a complete unknown, but it at least gives us a possible route to some kind of explanation as to what happened to Starbuck. Of course, this leads us into a whole new set of questions. The most salient of which being, of course, what the hell has happened to Earth?
I've spent the last five years of my life  praying that when the Galactica found Earth, it wouldn't be in the past (I've never been worried about them turning up in the present day, I don't believe there could be a TV writer left in creation that wouldn't realise that that was the mother of all crap-tastic ideas). People have been talking about this possibility (most chillingly over the idea that Baltar might end up becoming Jesus, which frankly would mark the first time the new series managed to drop the ball so badly Galactica 1980 would become a better post-Earth model), so I'm glad we've gotten past that. Obviously the new question is: who nuked Earth?
Again, there are a number of possibilities. One thing that bears thinking about is whether or not the Earth was in this state when the Final Five visited, or did the attack happen some time after. There are advantages and disadvantages to both scenarios. If we assume the Earth's destruction happened comparatively recently, the most plausible scenario is that the Ones, Fours and Fives found Earth first (hardly impossible, the implication given by the show so far  is that they were winning the Civil War, so they likely have more Basestars around to search for Earth), and blew the crap out of everything. I'm not convinced by this idea, though, mainly because finding a ruined Earth after years of searching is bad enough, and I think finding out you were just too late to do anything about it might be too dark even for Galactica.
So let's assume the attack occurred prior to the arrival of the Five. This ties in neatly to the "All this has happened before" idea, the idea that Earth was obliterated in a nuclear holocaust just like the Twelve Colonies were. The survivors of that war presumably relocated to Kobol, their records of the war became myth (this would explain the Scrolls of Pythia's impressive hit-rate predictions wise), and the Colonials got it into their heads that Earth was another colony, rather than the homeworld.
As well as tying in nicely thematically, it also allows you to have fun regarding who the Colonials actually are. The theory that they might actually all be Cylons, having taken on human form and wiping out their masters on Earth millenia ago, has been hanging around for quite a while now. I was never a huge fan of this idea, for two reasons. The first is based on the shows central concept regarding the cyclic nature of time; if a fleet fled Earth after a nuclear war, just like they have from Caprica (and for that matter New Caprica) it just makes more thematic sense for it to have been humans each time. The second reason is simply that it might be a bit harder to summon up sympathy for the humans if we discover they are in fact artificial life-forms who destroyed the actual human race generations ago. Again, this show is hardly one to shy away from dark and depressing, but "actually we're all dead and we've spent years following the fates of our killers" is probably pushing it.
No, I think it far more likely that if Kobol was the new home of refugees from Earth, that group would also be humans and not skin-jobs. The other advantage to this line of thinking is that it very easily lends itself to an idea about what the final ten episodes will concentrate on: namely, what happened to the original Cylons (or whatever they are), and where the hell are they now?
I shall post my thoughts on that very topic a little later, since I think this particular piece is quite long enough for the moment. Until then: keep watching the skies...
(Oh, and can I mention how much I love youtube? I was sat here trying to remember exactly what happened in the final scene, before I realised that I could probably just go there and type in "Galactica finds Earth". Sure enough, this led me to the final few minutes of the episode (also an "alternate ending", which is the same as the original ending but with U2 warbling over it; I never ceased to be amazed by what people spend their time doing), which was a big help).
 I did other stuff too, obviously.
 I spent some time defending the show over the whole Cylon civil war idea. It got quite a lot of flak for just how little air-time it got when it first kicked off. I counseled patience, assuming we'd get to see more of it later, but although we did get a little more info, there are a spectacular number of questions left unanswered. Did the whole fleet end up joining in? Did every base-star end up populated only by Ones, Fours and Fives or by Twos, Sixes and Eights? What happened at the Hub; did the One's shut down the resurrection capabilities for the rebel models? One of the most frustrating aspects of this show is the tendency to portray Cylon events as happening to individuals, rather than to an entire race of clones. The interchangeable names, for example. D'Anna was just a label a Three gave herself so that she could infiltrate the Colonials. There's no reason to believe that model was also the one who started going nuts on New Caprica, certainly she must have been different to the one in Downloaded (which gave us the first indication that the Three's had the potential to be pretty fucking dangerous), but for some reason that's the name they keep using. You have the same problem with Conoy: the original Conoy almost certainly can't be the same one that met Starbuck in Flesh and Bone, since it seems reasonable to assume that once the fleet was up and running (in both senses) the Cylon's weren't able to keep adding new infiltrators into the fleet. So given the current Conoy seems to be deliberately implied to be the same Cylon each time, why would he take that name? What were all the other Conoys doing while he had Starbuck held hostage in the bunker-of-love ? Season 2 made it clear that the Cylons were beginning to develop individuality (the implication is that this only happens when a model spends enough time with the humans for it to have some kind of impact), but the repeated tendency to focus on Conoy, D'Anna, and Cavil makes it hard to extrapolate to the rest of Cylon society. In fact, when was the last time we saw a Two that wasn't Conoy? Or a One that wasn't Cavil?
The inevitable side-effect to all this is that events that more or less make sense in microcosm (such as ambushing the rebels in The Ties That Bind) become desperately implausible on a wider scale. For example, given how Machiavellian, ruthless and bloodthirsty the Three's are it's hard to imagine boxing her entire line could have been easy, but we see Cavil put "our" D'Anna to bed and then just assume everything else went off OK everywhere else. We're being asked to believe that the Cylon's are sufficiently homogenised (I've been using that word a lot lately) that we can view a small portion of events and assume it can be applied on a wider scale, but the more time we spend with the particular individuals in question the more difficult that becomes to swallow.