From The Fix:

Pennsylvania Sen. Arlen Specter will switch his party affiliation from Republican to Democrat, according to sources informed on the decision.

Specter's decision would give Democrats a 60 seat filibuster proof majority in the Senate assuming Democrat Al Franken is eventually sworn in as the next Senator from Minnesota. (Former Sen. Norm Coleman is appealing Franken's victory in the state Supreme Court.)

Specter as a Democrat would also fundamentally alter the 2010 calculus in Pennsylvania as he was expected to face a difficult primary challenge next year from former Rep. Pat Toomey. The only announced Democrat in the race is former National Constitution Center head Joe Torsella although several other candidates are looking at the race.

I believe the only appropriate comments at this point are "wow" and "I guess we really will get to see if Pat Toomey is a more viable general election candidate than Snarlin' Arlen after all."

UPDATE: I have a few additional thoughts, as this begins to sink in. Some on the right will now declare "good riddance" or "he was a Democrat anyway." Perhaps this is true. But I think many will live to regret this. As a member of the Republican party, subject to the threat of a primary, conservatives had at least some leverage over Specter. Hence the flip on EFCA, hence the "no" vote on Obama's budget, hence the effort to shepherd numerous Bush appointees through the judiciary committee. Now that leverage is gone, and the pressures on Specter will be the opposite. Expect to see real changes in Specter's voting record. In fact, we saw a similar result with Senator Jeffords. As a Republican, Jeffords had a voting record on the left fringe of the Republican party, but to the right of every Democrat save Zell Miller. As an Independent, he was on the far left of the Democratic Party. A similar effect occurred with Democrats who switched to the Republicans in the 1990s. Republicans may enjoy losing an occasional thorn in their side, but the prospects for Obama's agenda have just been greatly enhanced.

The other thought is that this switch seems unlikely absent the primary challenge from Pat Toomey. In effect, Specter's chances of being re-elected as a Republican were close to nil no matter what; Specter's choice here is the rational one. But this presents a long-term problem for the Republican party. American politics are played between the 40-yard lines. But when a substantial portion of the party demands that their players occupy territory somewhere around the 10-yard line -- even in states that are positioned more around the opposing 40, how does that party ever claim a majority? So long as a substantial number of Republicans insist on positioning themselves between their own goal line and 40, they can't expect much other than Democratic supermajorities in both houses.