It's a known issue on the G4s and it was covered under apple care. If you had it, it's probably expired by now, but I would still take the iBook to the apple store and be really nice and they will likely fix it for you. They KNOW it's a problem although why it took so long for yours to start wearing off is a bigger mystery.
FYI, if you end up buying a new keyboard, which you can on ebay for $40 or less, the G3 is the same size, but the ribbon curves at a different angle and therefore won't sit flush, you'd have to kink the ribbon a little. The G3 keyboards are 100% better than the G4s, and the keys are not interchangeable. But FWIW, the G3s will work.