Mac 101: Reclaim your function keys – The Unofficial Apple Weblog (TUAW)
The folks at Bitbox have described the simple steps needed to get some of that old flexibility back. First, launch system preferences and click the “Keyboard” tab. Next, select the “Use all F1, F2 etc. keys as standard function keys” option.
For Excel lovers, this gets you back to using the function keys the right way and also check the System Preferences/Keyboard Shortcuts and turn off F8-F12 in the Dock and Expose section
