Can we put a moratorium on the phrase "filibuster-proof majority"? Yes, Arlen Specter's defection means that Democrats will hold sixty seats in the Senate once Al Franken is eventually seated. But the phrase "filibuster-proof" falsely implies that the party will have no problem passing its agenda through the Senate. In reality, as more sophisticated pundits have pointed out, getting sixty votes will still be very difficult on many issues.
Before Specter's defection, Democrats needed support from all of their caucus, including moderates like Ben Nelson and Evan Bayh, plus two Republican moderates like Specte, Olympia Snowe, and Susan Collins (with the required number of GOP votes increasing in the number of Democratic defections). That calculus is only slightly more forgiving today. While political science research does show that party switchers tend to substantially alter their voting behavior -- an effect that should be even more pronounced given that Specter faces a Democratic primary next year -- the net effect of Specter's switch is likely to be, say, a .4 vote boost on cloture votes for Democratic legislation in the Senate (i.e. Specter votes with the Democrats 75% of the time instead of 35% or whatever). The optics of the switch may be symbolically important, but contrary to Ron Wyden's claim, the legislative consequences are not likely to be "game-changing."