Some pretty tight analysis from Steve Benen:
...talk of a "filibuster-proof" Democratic majority is a stretch. For one thing, Norm Coleman just received a powerful reminder incentive to keep his legal fight going for as long as humanly possible. For another, the Democratic caucus, even at 60, still has Ben Nelson and Evan Bayh to consider.
But if reaching the 60-vote threshold doesn't make Arlen Specter's big switch "huge," what makes today's news a seismic political shift? It's further evidence of a Republican Party in steep decline, driven by a misguided ideological rigidity. Indeed, Specter suggested as much in his statement: "Since my election in 1980, as part of the Reagan Big Tent, the Republican Party has moved far to the right."
Basically. It's much more significant for the GOP than the Dems. I think getting 60 will still be a challenge. But Republicans face a more existential problem--not becoming the party of "We wuz robbed."