Well at least you found out that way. I bought Guitar Hero II before I found out you couldnt hook up PS2 controllers. And since I bought it from Circuit City I couldnt get a refund so I had to sell it on eBay. Lesson learned.
so why not have the ps3 controller...then switch over to the ps2?
Because the controller does not get recegnized at all until you press the PS button on that controller and guess what? the dualshock 2 dont have one :s
Sony will be fixing this in May when they release their PS2-to-PS3 adapter. After all, how would Sony make money if you could just buy any old adapter and use it on the PS3?
The nyko ps2-to-usb adapter has already claimed to fix this, and nyko has finally updated their website with information. They specifically mention guitars working with guitar hero.