Play it safe.

Apple Joins in a Recall of Batteries - New York Times:

As was the case with the Dell batteries, the problem is contamination by metal particles inside the lithium-ion battery cells. The microscopic particles are able to bridge a porous separator between the positively and negatively charged parts of the cell and cause a short circuit. This sets off a chemical chain reaction that can lead to the release of hot gases and liquids, smoke or fire.

The Apple battery recall (see Apple's web page here) applies to 12“ and 15” PowerBook G4s and 12“ iBook G4s. (Note it does not apply to any of the new MacBook line or to iBooks or PowerBooks of other sizes.) Battery replacement is free; look on the Apple web page (above) to see if your specific battery is included in the recall and sign up for a new one.