I recently had an experience where case stained my iBook (seems the G4s are made of an absorbent plastic).
I couldn't get it off with the standard approved iBook cleaners, so I took it to the Apple Store.
They tried a product called "Simple Green", and it worked great! They used it in it's concentrated form, and it couased no damage. it was a wee bit sticky afterward, but I just used a bit of iClean, and it was as good as new. I think it's probably a bit extreme for everyday needs, but for those emergency situations, it worked great.
Because I only used it on the outer case, i would be hesitant to use it on the keyboard, track pad, or any of the plastics exposed when the iBook is open. I would also strongly advise against using it on the screen.