I'm going with this. Correct me if I'm wrong, but didn't Sakurai not reveal any locked characters for Brawl until after it's release, because he considered them "spoilers"? This excludes Sonic and Snake obviously, as they had to be revealed beforehand for hype and all that jazz, with third-party characters being a big deal then.
With that in mind, I'd assume that every character we've seen so far (aside from Sonic and Megaman) will be starters. The only perplexing thing about this though, is that it would make Luigi a starter, despite him having been locked in every previous game.
But, if this speculation is true, Pikachu, Charizard, Lucario, and Greninja will all be the Pokemon starters, with Jigglypuff and any others that may be joining (or re-joining, in Mewtwo's case) being locked.