Statement from Specter's office is here.

In case you're not up on politics, that means that once Al Franken is seated, the Republicans won't have enough votes to block Democratic bills via fillibuster.

He does say he "won't be an automatic 60th vote for [breaking filibusters]", but that's still a big change from being a guaranteed "no" vote.

It remains to be seen how left Specter tacks to ingratiate himself with his new party, but this is huge news.