Check out the interview with Peter Jackson and Guillermo del Toro they explain it: http://www.slashfilm.com/2008/05/24/peter-jackson-and-guillermo-del-toro-answer-hobbit-questions/
It is going to be filmed at the same time as another Tolkien based movie (I can't find what it is, perhaps something from the Tolkien Reader?) and The Hobbit is going to be released in 2011. Based on...
Not yet, but it is coming. It is actually going to be two movies. The Hobbit is currently scheduled for release December 2011, with its sequel following in December 2012.