In an interview with The Hill, Arlen Specter plays with the idea of running as an Independent in 2010 and caucusing with the Republicans, sort of a reverse Lieberman deal. He'd have to stay out of the GOP primary if he did this, because Pennsylvania's laws don't allow primary losers to run in general elections.
I'd be really surprised if Specter went independent in this way -- it would create a highly unpredictable 3-way race and give him a bigger chance of losing than any other option I've heard. This is the one scenario in which Pat Toomey has a serious chance of winning, though I'd probably give the Democrat the best shot, and the race would depend on a bunch of stuff that I can't predict. So I'm guessing that Specter is playing some kind of funky game here. I don't really have a good idea what that game is, though.