So, here we are, thirty minutes away from learning the identity of the Eleventh Doctor. I must admit, I'm not nearly as curious about it all as I might have expected to have been. I guess there's two reasons to this. Firstly, I've already spent much of the time I allotted for wondering about Who's future considering whether Moffat is likely to drag the show kicking and screaming from its status as the number one logic-defying-breathtakingly-manipulative-roller-coaster-for-idiots show on television . Certainly, I consider it a far more important portent of the shows development (or otherwise) that Moffat will now hold the reins than Tennant is being replaced by, well, whomever. Doubtless the actor chosen will flavour the character to some extent, but since that flavour is probably at least partially a pre-known quantity, and is being selected by the show runners themselves, even today's choice is only particularly important insofar as it might give us an idea of where the show is being steered.
The second reason is connected to the first. Lawrence Miles has written at length about the absurd apotheosis of the Doctor the recent iteration of the show has forced down our throat, as well as attendant problems such as making the last of the Time Lords an attractive and athletic character. Perhaps "problems" is too harsh a word, at least without further discussion, but at the very least Miles is right that the producers have painted themselves into a corner. The idea of choosing Jon Pertwee to play the Doctor these days is laughable, irrespective of how much Venusian Karate kicks he tries to launch at his enemies. Even Tom Baker wouldn't likely have much of a chance, and that's a pretty terrifying realisation. I am fairly confident that you will be able to take whomever is revealed this afternoon, mentally paste them over David Tennant in every episode from New Earth onwards, and find that 95% of the time at least Doctor XI doesn't particularly seem out of place.
Still, there's speculation that they might be choosing a black actor this time around, which would be interesting. While I've always been less than keen on the idea of a female Doctor (though a spin-off show featuring a female equivalent would be something fascinating to see, which may or may not have been an idea discussed during or after Georgia Moffet's appearance in The Doctor's Daughter), I don't see any reason why a regeneration wouldn't increase the amount of whatever Time Lords use as their variant on melanin. Precedent has already been set, after all, with Cho Je in Planet of the Spiders. Hopefully that idea won't be followed too closely, though. Patterson Joseph as the Doctor sounds like it could work, but a black-and-white minstrel could do more damage to the show than even Adric managed.
Update: Huh. Never heard of him. Still, from the comments already springing up (which range from "wait and see" through "YUM!" "too much like Tennant" to "I feel physically sick", which I would suggest is something of an verreaction to any news not including a body count) it looks like I was right, you could replace Tennant with Smith and it not be too much of a hard sell. Still, maybe time will prove me wrong, from the snipped we saw of Matt Smith maybe the Eleventh Doctor will be an excitable stick insect playing maracas in The Cure. That, frankly, I would like to see.
 For the record, I think he will to some extent, purely because no matter how close he promises to stick to RTD's template, some change is inevitable because Moffat isn't a talentless buffoon, and all evidence suggests the man writes scripts on a computer, rather than daubing flashcards with finger-painted giant robots and flinging them at the DoP. I'm running out of opportunities to put the boot in as regards Who's Jabba-in-chief, so let me just point out that Macbeth described life as "a tale told by an idiot, full of sound and fury, signifying nothing." If he'd added "Also, it's filled with half-cooked Messianic allegories and pathetic character comedy mis-fires, and had Kylie in for no fucking reason at all", he could have been describing Voyage of the Damned.