The hole in the new Iraq strategy

Consensus seems to have settled on a surge and train kinda strategy for Iraq, but the whole thing is premised on a pretty big assumption.

Though a temporary increase of about 20,000 American troops is under consideration, the plan envisions the additional troops staying only until security conditions improve. After that, troop levels could come down, as better-trained and equipped Iraqi units took on a larger security role.

What happens if security conditions don't improve? To accomplish this "surge," the already overstretched military is going to have to use some troops scheduled for future deployment. What happens six months down the road when the situation is the same (or worse) and we've already spent those troops?