As reported in the Huffington Post, Obama instructed the CIA to step up efforts to find Osama just 3 days ago on April 28, 2011.
http://www.huffingtonpost.com/gary-s-chafetz/post_1988_b_854...
According to the article, Obama was concerned that the Taliban might gain political influence in Afghanistan after U.S. troops leave and if so, they may invite Osama back to the country. The believe is that killing Osama will help prevent the Taliban from regaining control.