Something I think is just about undeniable at this point is that we have a president who has decided that "leaving Iraq=losing." Even aside from whatever reason we're really there for, or what our long term ambitions in the region are, he's settled into this little narrative that can't be broken. Iraq is the central front in the war on terror. The terrorists are attacking us there. The terrorists want us to leave. If we leave the terrorists win.
We are not going to see significant troop withdrawals in '06, '07, or '08 absent a major rebellion by Republicans in Congress, which I can't see happening. Even a Democratic sweep in '06 won't help end this thing.
All the talk of benchmarks, timetables, withdrawal, whatever is somewhat moot. There's nothing which will cause Bush to call for withdrawal. It's his mission, his purpose, his raison d'etre. It's all he's got, and he's not going to let go no matter how many people die.