Ah, that's the rub. However Jackson has given us an answer - he will include stuff not found in the Hobbit; such as content from Tolkien's middle earth history and other stories.
That would make more sense, then. As long as he keeps with Tolkien's content rather than thumbsucking his own. While Tolkien used a totally different writing style with LOTR than with Hobbit, he kept the world consistent.