Steve Coll, as usual, gets to the crux of it:
[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. Clarity about this problem is important because if it is not recognized, it can’t be solved. I am willing to accept the possibility that the overall benefits of announcing a specific and provisional transition date outweigh the costs in this case—but there are costs, and these must now be managed. If Pakistani generals do not ultimately conclude that it is in their interest to abandon the Taliban and like groups as instruments of statecraft and national defense, then the risk-taking and fortitude on display in Obama’s decision this week may be defeated.
Defense Secretary Robert Gates seemed to acknowledge this danger during his Senate Armed Services Committee testimony today. ""We must not repeat the mistake of 1989 and turn our back on these folks," he said. That's a particularly compelling argument coming from a man who was a senior U.S. government official at the time. You can read more about that here.
(Also, I've done a bunch of Twittering on these issues, which you can peruse here.)