I feel underqualified to talk about this one because I don't claim any expertise in the area of foreign policy and have rather ambivalent feelings toward American involvement in Afghanistan. But Obama's speech tonight, whether or not it produces some near-term move in Obama's approval ratings (and it could), is really not the big news of the day. Rather, it's the commitment the White House announced earlier to beginning to withdraw forces by July, 2011 as a condition of the surge.
Politically, this seems very risky: in the long run, there's much more downside to breaking the promise than there would be upside to keeping it. If nothing much has changed in Afghanistan and our troops aren't getting out 20 months hence, we can presumably expect some major blowback, especially from liberals -- a primary challenge from Obama's left flank would not be entirely out of the question.
Of course, it may be precisely because the withdraw timetable is so risky politically that it is in fact credible; a credible withdraw deadline is almost certainly better than a non-credible one, but whether or not it's better than not setting a deadline at all, I don't know. I certainly do hope that Obama set the deadline to achieve policy goals and not to quiesce liberals -- if this was intended purely as a political move, it was probably short-sighted.