Talking Points Memo is reporting that Democratic Congressman Joe Sestak will challenge Republican-cum-Democrat Arlen Specter in the Pennsylvania Senate Primary. TPM has very few details at this point other than that Sestak is planning a run. Regardless of the final outcome, Pennsylvania's Democratic voters will be getting the opportunity to choose a strong candidate who was a Democrat without needing a poll to push him into the party.
Edit:
I just posted this to get the info out there, as I'm at work and haven't the time to get into it properly. Someone with more time please take a deeper look at this for a proper diary.
Update: TomP has a more detailed look at the news and what it could mean here.