Yes, it might have been his only route for political survival. But he'd been a Republican for a long time, and even stuck his neck out by coming out against EFCA (a position which, bizarrely, he apparently is sticking to, which will make the Democratic primary as hellish as the Republican one would have been). I genuinely expected he would have retired first. But the modern day GOP is so infuriatingly alienating to his moderates that I guess Specter felt he had to take a stand.
Once Al Franken gets seated, this is a 60 seat majority in the Senate (although Specter says he wouldn't be an "automatic" vote for cloture). I assumed we'd have to wait until 2010 for that. But thanks to intractable Republican extremism, Channukah comes early.
Specter's whole statement can be found here.