Apple has released a firmware update [1] for the 2009 Aluminum Apple Wireless Keyboard that improves battery performance when using them in conjunction with other Bluetooth devices such as the Magic Mouse or some Bluetooth headsets. Apple notes that your keyboard must be connected or paired to be able to perform the update. More information [2] regarding installation instructions is available on Apple's Web site. The update is available via Software Update and the Apple Support Downloads page. (Free, 1.14 MB)
[1]: http://support.apple.com/kb/DL1003
[2]: http://support.apple.com/kb/HT4019