Keyboard shortcut for toggling between Fn and F keys
Depending on what you're trying to do, you may be able to cut out the keyboard shortcuts altogether and completely automate this.  KeyRemap4MacBook allows you to set key re-mappings based on what application you're currently using.  

I am using a setting that switches F1 through F12 to standard Function keys whenever I'm using RDC (Windows Remote Desktop).  Any other time they operate like their icons (brightness, volume, play, etc).