the reason it doesn't work I have concluded is because it doesn't have the analog button that a controller has ( the button in the middle that lights up red and activates the analog sticks). When this is pressed on a regular controller it lets the PS3 know theres a controller there. Since the Guitar doesn't have one you can't use it because the PS3 doesn't know it's there.
EDIT: I have found a link that all ready states this. I guess I am a little behind.
http://ps3.ign.com/articles/745/745329p1.html also states that the GH guitar won't work. lol.