According to the official Hobbit movie blog, two movies will be shot simultaneously, with Peter Jackson as one of the executive producers. Also, speculation is rather pointless right now, considering the hoped release dates are in 2010-2011.
Overall, this seems like good news to me. A good adaptation of Hobbit has been a long time coming. But I never thought two movies would be necessary. LOTR is usually sold as three books, and each book is dense. I've always considered Hobbit to be the "gateway drug" for LOTR: less intimidating, generally not as potent/complicated (look, fewer subplots!), and hints of what the heavier stuff is like. Looking back at how the adaptation worked for LOTR, massively cutting here and adding there until we had three long movies (longer if you opt for the extended DVDs), I'm worried what will happen this time around, when the problem is reversed: not enough story for two movies. Will it turn out like King Kong?
Yet another problem will repeat itself for the screenwriters: the choice of which scenes belong in which movie. In retrospect, Two Towers may have been diminished because of the parts that shifted to Return of the King. I'm curious where the dividing line will fall for two Hobbit movies. The book's sense of scale shifts toward the end; onward from when Smaug appears, the book is suddenly no longer about just Bilbo and dwarves, but disagreements between peoples culminating in a Battle of Five Armies. Cut the story off too soon, and the first movie won't have much of a climax to speak of. Cut the story off too late, and the second movie won't have much plot left to cover at all. This should be interesting to observe.