If you have a PS3, you're fucked and have to buy new controllers; the official ones don't work. If you have the original games, you can't play them on the PS3 with the new guitar either, so you'll probably wind up having 4 god damn guitars kicking around (in addition to needing to have your PS2 hooked up to properly play the games anyway because of the bullshit third party controller security that those hackjobs at Red Octane put in there to fuck consumers over).
If you're on a 360, you're probably used to getting fucked in the ass on overpriced downloadable song packages. Can't wait to see how they manage to rape GH3 owners and make 'em like it. It's not horse armor-bad, but it's still pretty fucked up.
If you've just got a PS2 then you should be ok. I think. Who knows? Let's hope it doesn't suffer from screen tearing this time around.