Here is what Peter Jackson had to say in his first question and answer session with AICN about a year ago...

Question - Another major question that fans have had, has to do with the decision to make “THE LORD OF THE RINGS” prior to “THE HOBBIT”. Describe what made you decide to go with the trilogy first, what made you step over ‘THE HOBBIT’ and do you think that after your filmic journeys into Middle Earth with this trilogy, do you think you will want to visit Bilbo and his adventures from the first book?

Peter Jackson - United Artists have owned the US distribution rights to THE HOBBIT for many years (but not the right to actually make it). I know that Miramax approached UA a couple of years ago about doing some kind of partnership, but was not successful. That made up Miramax's mind to do THE LORD OF THE RINGS first.

I am happy to be doing the trilogy first, since it is much more complex and interesting than THE HOBBIT. THE HOBBIT has a very simple story with very little character development. It would actually be harder to adapt into a satisfying movie than THE LORD OF THE RINGS (and that has not been easy!).

New Line Cinema have the right to make THE HOBBIT (UA still owns the distribution rights), and have every intention of doing it following the trilogy. I don't know if I would be involved. It depends on my mental condition in 3 years!

And this is from his second Q&A In January

Question - As you've whittled away at the characters and storylines to complete your script, were there any characters or storylines set aside whole, to an extent that they might themselves serve as a framework for sequel/prequel/parralelquels for yourself or another film maker if these 3 films prove to be a wild success.

Peter Jackson - Not really. The key events and characters are all there - at least in my mind. Of course the depth of Middle-earth is so great that there are many imaginative ways to create sequels (beyond the obvious Hobbit prequel).
One idea I've got (if the trilogy is successful) would be to gather the cast together again and shoot another couple of hours worth of scenes to flesh out The Lord of the Rings as a more complete "Special Edition". In other words, we would write and shoot the Tom Bombadil stuff, or scenes involving Gandalf and Aragorn hunting Gollum, and his capture by Orcs ... and any number of other bits of business that we can't fit into the 6 hour version. That would be a really cool way of creating a "sequel" - expanding the existing The Lord of the Rings from 6 to 8 or 9 hours! It would be the first time that has ever been done (except for CE3K perhaps).
Anyway - one step at a time!

Question - for an example.. Frodo's adventure reaching Rivendell, Bilbo's HOBBIT story of course, and Aragorn's adventures in his youth.

Peter Jackson - New Line will definitely make The Hobbit if LotR is a success.