Macs do have a problem eating batteries ... they have for a while, its an unspoken truth that we all bury deep down inside...
The good news is that everytime its happened to me Apple has replaced the battery no questions asked. We've got a MB thats on its 2nd battery and that one is no longer charging... time to get them to send it off I think.
I had a G4 powerbook that did the same thing, it was out of warrenty and they took care of it.