That Bush has promised to veto. Pelosi must have gotten the support she needed. The Washington Post was reporting earlier today that both the anti-war and the Blue Dog Democrats were unhappy with the proposal. They must have agreed to the compromise:

In a direct challenge to President Bush, House Democrats unveiled legislation Thursday requiring the withdrawal of U.S. combat troops from Iraq by the fall of next year. The White House said Bush would veto it.
Speaker Nancy Pelosi (news, bio, voting record) said the deadline would be added to legislation providing nearly $100 billion the Bush administration has requested for fighting in Iraq and Afghanistan.

Bush was quick to respond:

Senior White House adviser Dan Bartlett, accompanying Bush on a flight to Latin America, told reporters, “It’s safe to say it’s a nonstarter fot the president.”
Talking to reporters aboard Air Force One, Bartlett called it “a political compromise in the Democratic caucus of the House aimed at bringing comity to their internal politics, not reflective of the conditions on the ground in Iraq.”
“It would unnecessarily handcuff our generals on the ground, he said. “Obviously, the administration would vehemently oppose and ultimately veto any legislation that looks like what was described today.”

I think it’s clear from the reports of the last couple weeks that Pelosi has been struggling to come up with a solution that will appease both sides, this is political, it’s not in response to what is going on in Iraq, which has been pretty positive.
Oh, and the Senate is doing this:

Pelosi made her announcement as Senate Democrats reviewed a different approach – a measure that would set a goal of a troop withdrawal by March of 2008. Majority Leader Harry Reid of Nevada called a closed-door meeting of the rank-and-file to consider the measure.

BTW, here is proof that there are some Democrats who don’t want to own this war and know that this will lead to it:

“It’s still micromanaging the war,” Rep. Dan Boren (D-Okla.) said.

Conservative Democrats fear the charge, still lodged by some Vietnam veterans, that that war could have been won had the politicians not intervened. More than anything else, many Democrats want to leave Bush responsible for ending the war he started.

“The war is the issue, but it’s the president’s issue, not ours,” Boren said.

But Pelosi and the other anti-war Democrats are determined to repeat history and own defeat (Rush’s favorite phrase) yet again.