For the last year or so, perhaps a little bit more, there have been rumors of battery problems with some MacBooks and MacBook Pros from Apple. As the issue did not rise to any real prominence, I figured there probably were not many units affected.
I guess I was wrong, as Apple has issued a support advisory on the subject, which you can access by clicking the link above.
Please note this is NOT the Sony exploding battery issue or anything like that.
The problem seems to have been in the OS X software, and not in hardware, though evidently some batteries could have been damaged and Apple will replace those, regardless of warranty status. They have a software fix available which supposedly takes care of all issues, as long as the battery is still good. Get it, or find out about your replacement options at the link above.