First of all, i don't have a PS2 controller, nor a PS2, and so i'm purely talking about hooking up PS3 controllers to Vista x64 via USB.
I think there's got to be more to the usb port than just charging. If i take my ps3 controllers to Cypher's house, my ps3 controller is still looking for *my* PS3 console. But the second i plug them into his PS3 console usb port, they are immediately recognized and functional. So, there has got to be some handshaking going on there. if i leave them plugged in, whether or not the wireless is being used is anyone's guess.