If running 9.1 on a USB Mac, or if you have 9.04(OEM, not retail/mail order/updater), you can write a script to do this. The script goes as follows:
tell app "Finder"
shut down
end tell
Save this script as an application. Go into the Keyboard control panel and click on the Function Keys button. Choose which key you want and attach the application you previously created.
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