Can you use ps2 controller on a PS3?

No, as nothing is forward compatible, though you could make millions if you could figure out a way to make ps3 controllers work for ps2, as well as ps3 games work for ps2's. Also, the ps3 controller is being updated to look a bit more like the ps2 one, as the lack of a rumble pack has gotten underneath of the skin of too many consumers. Also, the sixasis controller movement sensor seems to be more of a gimmick as it has yet to be successfully used in a hit game.