According to the Empire Online interview with PJ and GDT (link above), the two movies will include all of the iconic moments in the book, The Hobbit, as well as being expanded to follow other events that occur ‘offstage.’ This includes the White Council and Gandalf’s comings and goings to Dol Guldur.  Pj: “We expanded out the universe a lot more so that we weren’t just staying with Bilbo and the Dwarves on their journey, as the book pretty much does. We started to expand some of what’s happening to Gandalf outside of that journey.” Things we know are included so far:

– backstory of Thrain, Thorin’s father

– Beorn

– Spiders

– The White Council

– Gandalf’s journeys to Dol Guldur

– The three trolls (Tom, Bert, William)

– Sauron ( including some of his history)

Useful Links:
Message board thread: What does a “sequel” mean?
GDT comment about the 2nd Film
(Also see Empire Magazine link in 1.1 above)