After thinking about this Specter switch for a little while, I'm left with one question:
Does the GOP accept returns?
I'm not the only one wondering how good for fellow progressives this move actually is. Greenwald is openly hostile, and with good reason - Specter's been a consistent anti-civil liberties vote throughout the "War on Terror," and I doubt that's going to change now. Kos is lukewarm too. Both note that having Specter run as a Democrat pretty much precludes any liberal/progressive candidate from winning the seat (barring a Lamont-style charge from the netroots, which I don't see happening).
It would have been much better for the Democrats if Specter had been defeated in the primary by far-right Republican, Pat Toomey, who would lose to pretty much anyone the Democrats nominated. Specter should beat Toomey easily in 2010 (but Toomey now has all the ammunition he needs to paint Specter as a dishonest hack, which could make the race interesting), but that means we're stuck with a Ben Nelson-style Democrat out of a state that should give us more of a Russ Feingold.
There are things Specter's switch helps with, of course. Expect nominations to go through a lot more easily, for example (hello, Dawn Johnsen). Specter's also a decent shot at a vote for health care reform. But I still would have preferred replacing him with a civil-liberties advocate and honest progressive in 2010 to being stuck with Specter through 2016.