After digesting a bit, here's my sense:
1) This is a huge deal psychologically and in terms of the media narrative. Both coverage and polling show the GOP is increasingly a marginalized party, controlled by its most reactionary, zealous members. This really furthers that (largely accurate) impression.
2) The motivation here is pretty clearly expediency: he was going to lose a GOP primary. No way around it. This is the best way for him to keep his seat.
3) Considering that's the case, I don't think the Democrats really owe him anything, in terms of the primary. He's a member of the Democratic party, but democracy doesn't operate with guarantees and a good Democrat (or more!) should primary Specter. If Pennsylvania Democratic primary voters are persuaded Specter shares their values, then he'll win, if not, not. Seeing as how Penn is one of the most heavily unionized states in the union and Specter still says he'll filibuster EFCA, I think Democratic primary voters should have the opportunity to vote for someone who believes in the human right to organize.
4) The basic power dynamics in the Senate remain somewhat unchanged. The fact is that the fulcrum of the entire agenda is a collection of about six or so self-described "moderate" senators: Collins, Snowe, Specter, Nelson, Conrad, Bayh, and it doesn't matter a whole lot what letter they have in front of their name.
All that said, they've got to be happy over in the White House, and pretty damn pissed in the GOP cloakroom.