I am totally for Petey being a playable character.
I've read this topic, and most people want Toad over Petey because he's "been with us longer". That's bull****. It doesn't matter how long Toad exists, he's not going to be playable in SSBB because he can't do a **** thing.
Petey Piranha on the other side, has lots of moves to his command. He can fly (Toad can't), he can vomit (Toad can't, only if he's eaten some bad food), he can create small tornado's (Toad can't), and he wears a red white-spotted speedo! (Toad doesn't)
I'd say, if any new Mario character will be put in Brawl as a playable one, it will be either Petey Piranha, Bowser Jr, or Waluigi. Waluigi is being ruled out pretty much, since he hasn't been in any story-related Mario game so far. If you think the Mario sports & party games have a story, they don't.
Now, i know Bowser Jr. played a bigger role in the Mario Sunshine story, but Petey's pretty much the only recent boss character who can make it into Brawl. I don't think that bosses like King Boo, Gooper Blooper, Mecha Bowser, Wiggler, and the Manta Shadow can make it into Brawl.
So, i'm totally for expansion of the Mario characters in Brawl with Petey Piranha, Bowser Jr, and Waluigi.
And DainBramaged, i think Petey is pretty popular, not only with the fans of Mario, but with Nintendo as well. Why else would he appear in lots of sports games as a playable character? In Mario Strikers Charged, he was made captain, not Toad.
Also, i can see Petey becoming a playable character in Mario Party 9. It's about time Petey made his entree there.