Steve Coll worries:
[A]n honest accounting of the decision to name the 2011 date should acknowledge that the specific date will certainly encourage some in the Pakistan Army to persist in their belief that the U.S. is headed to the exits in Afghanistan and that they, therefore, should persist in their hedging strategies toward the Taliban, to protect their interests in the aftermath of a U.S. withdrawal.
Von at Obsidian Wings puts the date to begin withdrawing troops in context:
A lot of Republicans support Obama's plan, but are pushing back against his deadline for withdrawal in 2011. I understand the argument against the deadline, but I think these Republicans miss the bigger picture. There is no popular support for an open-ended commitment, either in the US or abroad. Without a deadline, there is no surge and no chance to convince allies to aid us. Obama's surge does not exist without a deadline.