From what I understand, the second film will somehow bridge the gap between The Hobbit and Lord of the Rings, most likely using Tolkien's unfinished works. I, for one, haven't read ALL of Tolkien's stuff.. I have the "big four", The Silmarillion, and Unfinished Tales, but I know there's a bunch of other stuff out there.
One problem that Jackson (or any director) will have is trying to get the feel of The Hobbit to match what's in the LOTR films. I'm going back and reading them all, and The Hobbit is very much written as a children's book, even though Tolkien would later incorporate those elements more firmly into the "real" Middle Earth that was created for LOTR.
I'm starting to think of the way The Hobbit was written... if they look at it as though the book that Bilbo wrote is an adaptation of Bilbo's adventures, and perhaps not a "true" representation of what really happened (as if Bilbo re-wrote his adventures as a children's tale for little hobbits), then a director could alter events to be darker and grittier while still maintaining the overall story elements. Because I personally don't think you could make such a "goofy" kid's movie in Peter Jackson's Middle-Earth.
Plus, he'd be able to incorporate other Tolkien elements into The Hobbit. In the Unfinished Tales book, there's one section that amounts to a "deleted scene" from Lord of the Rings, where Gandalf details the precise reasons why he sent the dwarves to Erebor (it was his idea, not Thorin's), and how it was he came to choose Bilbo as their final companion. Very interesting read, that.
I think the "Why Jackson?" argument has already been answered.. consistancy with an already-created world. But also remember that before these films, most people thought that LOTR was unfilmable, that it could never make the transition to the screen. While some story elements were changed here, and there, I do think that Jackson did indeed do the impossible. That's the greatest compliment I can think of to Jackson's work on LOTR... "You did it."
