According to an interview with Gametrailers transcribed by Gamecenter, there will not be any DLC for Super Smash Bros. Brawl.

Gametrailers TV: GT user TwilightKite wants to know about downloadable content, and what we can expect.

Masahiro Sakurai: I’m sorry to say that there isn’t going to be any downloadable content available for the game, however we do have user generated content that will be passed around. For example, you’ll see custom built stages coming through the internet to you.