It has been confirmed that trailers for the following movies will be shown preceding The Dark Knight:

Body of Lies

Watchmen

Terminator 4: Salvation

On a note about Watchmen, as reported by The Geek Show Podcast, the Watchmen trailer will be attached to prints of The Dark Knight one week after it opens. However, my guess is that it will probably show up online around the same time, if not maybe a little earlier.