‘December 16th is going to be a big day in genre films, with looks at both The Dark Knight Rises and The Hobbit: An Unexpected Journey in conjunction with major theatrical releases, according to Collider.  The Dark Knight Rises is the big one; a six to seven minute “prologue” of footage from the film will appear at some Imax theaters before showings of Mission: Impossible—Ghost Protocol
A teaser trailer for The Hobbit: An Unexpected Journey will launch before showings of Sherlock Holmes: A Game of Shadows.  A new trailer for TDKR will also be shown with the Sherlock Holmes sequel.  
There’s been video from The Hobbit before, in conjunction with the production blog (see “Peter Jackson’s ‘Hobbit’ Production Blog #4”), but no trailer.  A teaser trailer for The Dark Knight Rises appeared with Harry Potter and the Deathly Hallows, Part 2 (see “The Real ‘Dark Knight Rises’ Teaser Trailer”).