I tend to stand behind IGN when they say that any character from Melee with an original Moveset will be in Brawl, so I think both Jigglypuff and Mewto will return, although I hope they improve their movesets because even though they were both servicable, they just aren't powerful/good enough to stand up to the like of Sheik, Fox or Marth/Roy (Not to mention all the others). Mewto should be a great character. He's a freakin' psychic! but for some reason they couldn't pull it off well. Partly I think because he had only one or two good B moves and very slow clumsy-feeling movements.
As for new Pokemon, I think Pichu was dropped for Pokemon Trainer and that'll be the last new Pokemon anything in the way of Playable Characters.
(s)he's obviously in the game....and probably the first unlockable character any of us are going to get...just like the previous 2 games
i'm personally hoping for pichu, what's the harm in letting him stay, some people preferred him over pikachu and that will just let those people down..