If we are to believe what could (until recently) be seen in a preview video on the Japanese Wii website, then indeed, Ness, Jigglypuff, and newcomer pokemon Lucario will be joining the Brawl in a few months.
The video in question was designed to introduce players to the new sticker system that will be incorporated into Brawl's adventure mode, but it also inadvertenly revealed the new characters. These stickers can only be used by select brawlers, and the sticker selection screen, of course, lets players know who they are good for. It does this with small icons depicting those particular characters.
In the video, which has since been modified, sharp-eyed viewers could pick out low-res images of Ness, Jigglypuff and Lucario in that information bar, so expect to wield baseball bats, sleep-inducing songs, and...Lucario (what does he do again? There are just TOO MANY pokemon) pretty soon.

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It's on the Groudon sticker... And Ness can use it? I don't think so. That's the Pokemon Trainer's head. Ness' head is on the Mother themed sticker though.
Nice to hear that Ness and Jigglypuff are in the game. They were my favorite fighters in Melee.
Good for Ness! I was about to say, have the dude from Mother 3 and not Ness?!? Absurd!
Isn't that a pichu too?
No, just Pikachu...