after reading what krugman said,1 it's worth repeating what i wrote last december. the 50 senator threshold that democrats needed lieberman to clear in 2006 when they sought to control the senate is not the same as the 60 senator threshold that the democrats now need to beat a filibuster.
the democrats put up with lieberman in 2006 because, by allying himself with the democratic caucus, he delivered the democrats control of the senate. deciding who controls the senate, however, is essentially a head count. it doesn't depend upon votes of individual senators on individual issues.
with specter included and assuming that franken is eventually seated, it's true that the democrats will have 60 members of their caucus in the senate. and it's also true that you need 60 votes to avoid a filibuster. but that doesn't mean that the democratic majority is filibuster-proof. a filibuster is decided on a case-by-case basis. it's not a one-time head count like control of congress is. senator specter (or any senator) is free to vote however he wants on each cloture vote. just having a "D" next to his name doesn't guarantee anything. indeed, specter has already said that he's not going to be an automatic filibuster-buster for the dems. as atrios says, specter is still free to be a dick. just like he always has.
1- and what paul campos says.