I really doubt they'd put in another character this late in the game, although it is possible.It must be something big......like a super pimpin 3rd party character.
The only thing I can think of that would delay the game that they couldn't foresee is...well...I guess the only thing I can think of is that they were having talks with Capcom, and they just now reached an agreement to have Megaman in Brawl (the company and character are both just examples).
So it seems the chances for a cool new third-party character (besides Snake and Sonic) just went up, at least according to this logic. Sorry for doubting you, Orion.
EDIT: Of course, Nintendo might just be like every other game company that can't make accurate release date predictions despite decades of experience...