I don't really believe it. If he supports card check, votes for cloture on a very regular basis (no matter his actual vote on whatever bill), and publicly supports Obama and his policies (for the most part) then I'll believe. Unless and until, it is just a ploy to get reelected.
From a bigger picture perspective, I'm not so cynical about this as others. Matthews was saying he's an opportunist, lots of blog comments about him not actually standing for anything but getting elected. I don't see how that's a bad thing though. Politicians are supposed to represent their constituents. Reps their district, Senators their state, and the President his (or her...someday) country. Rep terms are short enough that there are plenty of referendums on their record by their constituents. In the time since Specter was last elected, however, the state of Pennsylvania has changed quite a bit, really to the point that, primary challenge aside, as a Republican it is not possible to represent PA well.
If the GOP did not require the ultimate fealty to party above all that they do, the point would be moot, but Republicans seem to consider disagreeing with the party line on something tantamount to treason. Despite being their nominee for president, McCain is still railed against by much of the right because he thinks torture is bad (Rush had a comment about Specter taking him with to the Dem side). Despite the fact that she is a bigger national punchline than W ever was (you know, before his being president became so tragic), they love Palin.
Flexibility in a politician is often seen as bad (waffling, not having principals, et cetera), but it really isn't, to the extent that a politician changes her views and votes to get reelected, she must be changing them to reflect the changing views of her constituents meaning that she is better representing those who elected her. Having a principled ideology is a fine thing, but not if you are claiming to represent a varied and changing populace, unless you don't mind finding yourself on the outs from them. Principled stands do have a place, and politicians who truly take them are quite courageous. They are also seldom reelected. If one's convictions align with the voters it doesn't take strong principals or courage to state them. If they don't align, he doesn't get elected.
I do not like closed primaries (as exist in PA) because they throw a wrench in the works. In the general election, the voters may not have their choice of politicians, because one party or the other may decide to nominate the more polarizing, divisive candidate. Primaries should be about each party sending the candidate that has the best chance to win the general. But to really do that, primaries should all be caucus type. Short of that, I like the notion of a general election as a choice between all candidates of any party. If someone gets 50%, they win, if not, top 2 go to a run-off. This gives everyone a say on every candidate. The big parties would fight like hell to keep this from happening expressly because 3 Dems, 4 Repubs could all split the vote so that the 1 Liberatarian and 1 Green candidate go to the runoff. Of course, that result is another reason I would like this.