I did this cheap PS2 to PS3 Guitar Hero 3 adapter a few months back, but only about a month after I bought my PS3.
So, I bought my ps3, with GH3+guitar. I made a custom PS2 guitar a while back so I thought it would be nice to use with the PS3. A cheapoo PS2-usb adapter worked with the controllers, but not with the guitar.
To not start from scratch, I found a PS2-USB adaper at http://vusb.wikidot.com/project:ps2usb but needed some "finetune" to work as a guitar.
First: mine only support one controller/adapter
Second: the HID descriptor had to be modified (copied what was possible - due to limitations of slow USB device class)
I've dumped the descriptor of my guitar so you can experiment with it, if you want. Download: USB HID_gh3_n2.rar
And at last: added a code to switch the controller to analog mode, so the whammy should work too. (you can connect before or after the adapter is powered, no special combo needed)
Source, binary and etc (avs include absolute path!) Download: Source.rar
Results: the adapter works great, fast, virtually no lag BUT i was unable to identify why the star-power is in constant active state :s
P.S.: it only works with PS2 GH controller (or analog controller with constanly pressed left D-Pad)
If I'm not wrong there is an adapter to convert Wii guitar to PS3. you can find it if you search a bit on Google. It's called Cross Play and is designed by Datel. Don't know how well it works though...
Codekiller i wish i could give you a metal for this because thats awesome (i wonder if we could wire over ps2 controllers over the same way?) if i hadnt imported the adapter along with the memorycard reader when i first bought the ps3 i would probably be jumping all over this and trying it out myself.