There's a lot to be said regarding Specter crossing the floor. Firstly, while I wouldn't go so far as Larison has in decrying this as a move entirely based on electoral politics, that's mainly because I don't know either Specter or the political situation in Pennsylvania well enough, rather than having any strong counter-evidence. All I know is that Specter was a Democrat in his youth, and has been, by and large, the closest the Democratic Party has to a Republican ally in the Senate.
Pure survival instincts notwithstanding, I am unsurprisingly sympathetic to Specter's claim that this is a move based on the recent direction of the Republican Party. Since the GOP have been drifting to the right since, ooh, Goldwater, probably, but certainly since 2000, it seems to be an immediate corollary that the midpoint between the Republican platform and the Democratic platform is also drifting right, which means that year by year more politicians are finding themselves on the left hand side of that dividing line. If Specter is claiming this has just happened to him (or perhaps that it happened some time ago but the line is now so far to the right that he has no choice but to switch parties), then I can't claim a priori that he's making it up. I think it was Zell Miller who said "I didn't abandon my party, my party abandoned me."
The interesting thing about Miller, though, was that he was a Democrat who defected to the Republicans. This gets me onto my feelings about crossing the floor in general. I am somewhat uneasy about an elected official changing his or her political party partway through a term (or in this case, a year or so from the end). The reason for this is obvious; the political affiliation of a given candidate is quite clearly a consideration when voting for them. Some will give it little or even no weight, some will consider nothing else, but as President Bartlett himself said on the subject of voter choice "We don't get to choose what's important". Whilst he claims that his priorities and voting tendencies will not change, declaring that that should be enough to satisfy his constituents is dodgy ground, I think, though he does admit as much.
On the other hand, as leery as I am with Specter becoming a Democrat (much as we needed him, and more on that later), I would be vastly more angry at someone emulating Miller. Partially this is tribalism, I grant you, but I think there's a case to be made that defecting to the GOP is less justifiable than defecting from it.
Firstly, the argument that one can switch parties without it changing the way you vote is less convincing when you're joining a party that votes in lock-step on many issues, and engages in some fairly vicious attacks when members don't play ball. Second, the "my party left me" argument no longer holds. The Democratic party has not drifted to anything like the extent the Republican party has, and more importantly, it hasn't drifted left. A Democrat choosing to become an independent might thus make sense, but crossing over to the Republicans cannot be sensibly viewed as the politician remaining still while the party moves away from them.
The third point to be made in all of this, of course, is that once Al Franken is seated , Specter will give the Democrats a filibuster-proof majority in the Senate. I think a brief explanation of the filibuster might be appropriate here, for the aid of those less obsessed with American politics than I am.
A filibuster is used to prevent a bill from being voted on by extending the debate indefinitely, through the means of simply beginning a speech which then takes up the entirety of the remaining time before voting (see the West Wing episode The Stackhouse Filibuster). In the US Senate, 60 "cloture votes" are required to force the closure of a debate, thus rendering a filibuster impossible. I'm not sure whether this was the original intention of the creation of the filibuster (if indeed it was a creation, rather than a loop-hole someone discovered), but the end result is that there exists a method whereby which a minority opinion can prevail over a majority opinion, given sufficient dedication and organisation (well, dedication isn't quite so much of an issue anymore, since nowadays the filibuster is usually threatened rather than performed).
Whether or not it is a good idea for there to exist a method by which the "tyranny of the majority" can be defeated is an interesting question. Certainly I recognise the advantage of ensuring that the majority doesn't automatically get everything it's own way, especially since the filibuster only works if you're fighting between 50% and 59% of the Senate.
What does seem clear, though, is that there is a world of difference between preventing the majority from always winning, and preventing it from ever winning. If the majority should not hold the minority hostage, it immediately follows that the minority should not hold the majority hostage either. That, though, is the current situation in the Senate. The Republican minority is holding the Democratic majority hostage. The chart below shows the increase in cloture votes.
Thus, while I'm not sure how happy I am with crossing the floor as a general idea, in this case I'm more than happy to accept that even if the people of Pennsylvania are pissed that they voted for a Republican and got a Democrat, they can at least comfort themselves with the fact that the Senate can actually start moving once again, on issues that the Pennsylvanians and pretty much everyone else want there to be movement on. In an ideal world, I would like the Republicans to use the filibuster in the extraordinary circumstances it was once reserved for, rather than on every single bill. In a slightly less perfect world, I'd like to either remove the filibuster or reduce the number of cloture votes required (or even change the number of Senators, which I always thought was a weird idea anyhow). Absent either of these things, though, I'll settle for rendering it a moot point, or at least a moot point on whatever issues Specter chooses.
Update: Eric Zimmerman digs out what Specter had to say when Senator Jim Jeffords left the GOP for the Democrats in 2002.
"I intend to propose a rule change which would preclude a future recurrence of a Senator's change in parties, in mid-session, organizing with the opposition, to cause the upheaval which is now resulting," Specter said. "[I]t is my view that the organizational vote belongs to the party which supported the election of a particular Senator."Interesting, huh?
 Assuming Norm Coleman ever shuts the hell up, obviously. This is one of those times where I recognise the man has the right to continue his court case, but at the same time he's a hypocrite and a bastard for doing so (back when he thought he was going to win by a nose instead of lose by one, he told the press whomever lost should not enter into a protracted legal battle, for the good of Minnesota), particularly since there are some suggestions he is deliberately dragging his feet so as to keep Franken out of the Senate.