Along with yesterday's release of OS X 10.6.2, Apple issued a small update for their wireless aluminum keyboards, which allows users to utilize the one-touch function controls. The update is specifically for 10.5.8 users, since Apple has included this update in the latest version of Snow Leopard for 10.6.x users.
If you are running OS 10.5.8 and are not able to use the function keys on your wireless keyboard, then this should address the problem. The update should be available via Software Update, but is also available as a 11MB download from this website: http://support.apple.com/kb/DL957