First I'd heard about it. I wonder if Jackson will do it, he's already producing it and he'd probably have a significant hand in it anyway. The reason given: startup delays, which means MGM's frailty. Jackson said: “...“The bottom line is that Guillermo just didn’t feel he could commit six years to living in New Zealand, exclusively making these films, when his original commitment was for three years. ...”
http://www.theonering.net/torwp/2010/05/30/36920-guillermo-del-toro-departs-the-hobbit/