Replacing the harddrive on the iBook will void your warranty, but not on PowerBooks, or PowerMacs. Pretty much if it's not in the user's manual and Apple finds out that you did it, they may deny service. However,doing something that you can prove in a court of law will not effect operation is a gray area.
Most car companies have warranty statements that essentially say things like: "If you install Turbo, we won't fix your suspension." The suspension, however is not effected by Turbo, so most courts have said that the companies have to fix unaffected parts under warranty.
So, if you doccumented the paint removal well, and were willing to spend a costly legal battle against Apple, the court would probably decide that Apple had to fix your iBook, but by that time, everyone else would be quantum computing.
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