Not quite sure how I feel about that, since a primary loss by Spector to Toomey would have made Penn. a relatively ironclad progressive pickup. But, on the other hand, He's really at the mercy of the Dems now; there's just no way he can return to the Republicans, so he's a Dem until he's out of politics.
Arjun Jaikumar at DK suggests that Snowe is next up. Perhaps. I'm more concerned about the Coleman/Franken thing. Now that the only thing between the Dems and sixty Senate seats is the resolution of that court case, Coleman will be under tremendous pressure to draw it out however he can. Above and beyond that, though, Franken is a progressive Dem. Progressives badly need that, because the Blue Dogs are going to try to assert dominance over the party's agenda now that they are the "swing vote".
They shouldn't get too comfortable. Progressive challengers are already being readied. But until then, progressives will need high-profile voices like Franken to ensure that the "centrists" don't take the helm.