We don't know who the other playable character is.
First off, though, we know that Luigi is a playable character in Super Mario Galaxy 2, and now we know how. The Official Nintendo Magazine in the UK has a review of SMG2, and in it they say that Luigi becomes playable once you find 20 stars. That doesn't sound too bad at all. Hold on, Luigi! We'll use Mario as a disposable character to get to you!
Get this, though: There's another playable character? ONM hints at another unnamed character for Super Mario Galaxy 2 when you reach 120 stars. Who could it be? Peach? That would be fantastic. It would be like Super Mario Bros 2 all over again!
GoNintendo's bulletpoints from the review reveal several other interesting things about the game. It seems that Bowser's stages are similar to those of Super Mario 64. Damn -- hated those. It seems that SMG2 takes place in an alternate reality where Mario and the Lumas never met. Here's some good news for me: there's supposed to be a good amount of 2D sections and several throwbacks to earlier games in the later levels.
[Update: Not to crush your Peach hopes, but it seems that 120 could get you something, though it may not be another character.The guys at Official Nintendo Magazine contacted GoNintendo to clear that up a bit. 120 does get you something, though. New rumor: Nude patch?!?]