@Fido,
Since I'm a Pennsylvania voter, and therefore a member of the demographic Specter's appealing to, let me say that he's the world's leftest Republican and rightest Democrat. A perfect moderate.
This isn't the first time he pulled this trick. Way back in 1980, he switched from the Democrats to the Republicans in protest over the Philadelphia Democratic machine's endemic corruption (Philadelphia hasn't had a Republican mayor for over half a century). Since then, he's been acclaimed for his moderateness, and somehow, when the national Republican party is in tatters, a victim of its own internal corruption (just like the Philadelphia Dems), it doesn't surprise me that Specter switched sides...again.
The question now is, how well will he do in the Democratic primary? Who will his opponents be? He didn't stand a ghost of a chance of winning the Rep primary, we can all agree, because at a national level, the Reps have been nominating more and more sacrificial lambs instead of taking the fight to the Dems on the issues. But what about the Dems? Hoeffel came close to beating him in 2004 IIRC (on a purely negative campaign, I might add); Rendell's gubernatorialship is soon coming to an end as well and
he might seek the senatorship; the only person I can think of who wouldn't challenge Specter in 2010 would be Bob Casey, our other U.S. Senator.