Looks like that's now the 2010 PA Democratic primary fight. About which Sestak, a former Navy admiral who represents the southeastern Philly suburbs, probably isn't so thrilled:

Earlier in the year, Sestak was not inclined to run against Sen. Arlen Specter (R-Pa.), but the possibility that Specter could lose a Republican primary to Pat Toomey and the lack of announced Democratic candidates prompted him to reconsider.

Update: Josh Marshall makes a good point, namely that it seems likely the Democrats will clear the primary for their new compatriot.

--Michael Crowley