To be more specific I'm running Snow Leopard (10.6.7) on a late 2009 MBP. I never really used Dashboard and disabled it using the terminal and now I have the F4 key not doing anything. That would be awesome if I could make a keyboard short cut such as "Hide Others" or "Show Desktop" or whatever. I have this program called BetterTouchTool which does keyboard and mouse gesture shortcuts pretty well but it won't do keyboard shortcuts using F# keys.
You have to turn on "F1, F2 keys as standard function keys". This is done in the Keyboard preference pane of System Preferences. It's the one in the middle. AFAIK you can't have some functions (itunes play/pause etc) on and others off.
On my keyboard I have F13-15 set up as Expose, Expose on current app and show desktop (by default I think). MBP doesn't have these, but you can set them. In the Keyboard prefpane, Keyboard shortcuts, Expose and Spaces. Set as whatever you can.
http://kevingessner.com/software/functionflip/ You pick what f-buttons you want to be able to use without the FN button.