FunctionFlip might be what you're looking for.
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FunctionFlip's purpose is simply to disable the special features — rewind, play, mute, etc. — on the function keys. For example, if you "flip" F7, F8, and F9, those keys — only those keys — will revert back to normal F keys. Press the fn key with the special key to get the special function back. That is, the "special" and "normal" functions are flipped.
To assign a custom shortcuts to your reclaimed keys, use a program like Quicksilver or Keyboard Maestro.
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http://kevingessner.com/software/functionflip/