In an article about 4-day passes to San Diego Comic Con selling out in only 3 hours, TheHDRoom said that Summit is expected to show footage from Breaking Dawn at the convention:
This year’s Comic-Con in particular is poised to be perhaps the biggest yet. In a battle of screaming female tweens versus comic book faithful, Summit Entertainment will present new footage and the stars of The Twilight Saga: Breaking Dawn Part 1 while Marvel and Paramount will counter with likely the first public footage from The Avengers.
You can read the full article at TheHDRoom. Cast members previously attended Comic Con to promote The Twilight Saga films, so doing the same for Breaking Dawn seems to make sense!