Main production on The Hobbit has not been greenlit because of MGM and their potential insolvency. They're well along in designing sets, effects, makeup, and costumes, but apparently everything's on hold. The article implies that they can begin production as soon as they get the OK, there's just no telling when that will be.
http://www.torontosun.com/entertainment/movies/2010/05/28/14173166-wenn-story.html
http://entertainment.slashdot.org/story/10/05/28/2049209/The-Hobbit-On-Hold?art_pos=11
http://www.torontosun.com/entertainment/movies/2010/05/28/14173166-wenn-story.html
http://entertainment.slashdot.org/story/10/05/28/2049209/The-Hobbit-On-Hold?art_pos=11