Apple has released a firmware update for 13" MacBook computers. The update addresses battery problems. According to the description from Apple, the update improves the computer's ability to maintain a charge when the system is shut down. This update applies to all MacBook models (Black and White as well as Aluminum), and is not for any Pro or Air models.
This update is not necessary for any MacBook that has a "Pack Lot Code" of "0003" (this can be looked up in "System Profiler" under the "Power" section).
The update should be available through Software Update, though can also be downloaded here.