No, you didn't void the warranty.
Take out your extra RAM, and see if it works then. Then pull out the Apple RAM and put it back in ("reseating"). Again see if it works. See what happens if you put both modules in, but in the opposite slot (this lets you see if it's the slot that's gone bad, instead of the RAM).
If you can't get it to see both, take the PowerBook to the dealer with just the Apple RAM and have them fix it.
tooki