Rep. (and retired Admiral) Joe Sestak (D-PA) is refusing to rule out a primary challenge towards Republican-turned-Democrat Arlen Specter (D-PA). Sestak has a stellar resume and has proven to be an effective campaigner, trouncing a 10-term (albeit scandal-plagued) incumbent to win his Republican-leaning seat in 2006 and winning re-election handily in 2008.
While the Democratic Party machinery has officially united behind Specter, and Sestak specifically admits he'd be bucking the party if he ran, I wouldn't be surprised if there wasn't some wink and nod collusion between Sestak and the party here, to put some leftward pressure on Specter and keep him honest. It does no good to have Specter as a Democrat if he refuses to join on cloture votes and works to water down Democratic legislation. The only thing that will stop him from taking that route is if he has to worry, at least a little bit, about a primary challenge.
Sestak says that his ultimate decision will be based on Specter's votes on health care, veterans' issues, and the military. I'm guessing his announcement is just meant to keep Specter conscious of his political position. If the new Democrat behaves himself, Sestak will probably drop out, mission accomplished.