It would probably take two films to really do the book justice. They had to take a LOT out to make the LotR films of reasonable length
I believe that tbe usual theory is that the 2nd movie would be based on events offstage of The Hobbit, such as the Meeting of the White Council, and events subsequent to that, such as the arrival of Sauron into Mordor, leading up to the LOTR.
Ecurbh, I think we need a Ring Ping!