From Eleanor Clift:

...Funny, yes, and seriously Machiavellian, claims one GOP source, who attributes Specter's surprise defection to a Biden-Rendell bank shot. Vice President Joe Biden is from Scranton, Pa., and Ed Rendell is governor of Pennsylvania, ties that put them solidly in Specter's orbit. Biden spent the last three decades in the Senate and knows Specter well. They served together on the Judiciary committee back in the day when they along with many others made fools of themselves grilling Anita Hill. The self-interest of the Obama-Biden administration in getting that 60th vote in the Senate is self-evident. It changes the calculus for everything going forward (health care reform, judicial nominations) if Democrats have a filibuster-proof majority in the Senate.

Then there's Rendell, whose personal ties to Specter are extensive, but who also has a political stake in seeing him hold his seat in Pennsylvania. Rendell once worked for Specter and regards him as something of a mentor; they're friends. For Rendell, a gregarious, ambitious politician, a Senate seat would cap his career as a former mayor and popular two-term governor. Those who know Rendell say he really wants the seat that Specter holds but would not run against his friend.