I'll also point out, that while it is clear that Americans are war weary, when the Taliban and al Qaeda, the reasons we are in Afghanistan, are included in the poll question, it tells a different story.
When asked how much of a threat to American national security a return of Taliban rule in Afghanistan would be, 91% of Americans polled said it was a serious threat (51% very serious, 21% fairly serious, 19% just somewhat serious). Only 8% said it was not so serious a threat. (1)
When the Taliban are included in a question asking about withdrawal, it also tells a different story. Americans say it depends on our progress. 62% say depending on military conditions or when the Taliban have been defeated. Only 18% say now. (2)
(1) (NBC News/Wall Street Journal Poll Aug. 5-9, 2010. N=1,000 adults nationwide)
(2) Question 33