First, I must give credit to my colleague for finding this today...
How do you shut down or restart your Mac when you are at the login screen and the Restart and Shut Down buttons are disabled? It's simple; just type >shutdown or >restart as the user name, then click the Log In button. As far as I can tell, this only works in Tiger.
[robg adds: While testing this one, I found you can also use >sleep to enter sleep mode...
Some may claim this is a security hole of sorts; I don't believe it is, as being able to shut down or restart from the login screen implies that you can actually reach that screen in the first place. If this is a public kiosk machine, the login screen will be unreachable. If this is a home machine, more than likely the power button on the CPU will be reachable, which would let one do the same thing (without being at the login screen, for that matter). I just think this seems like a very handy timesaver -- note that you must have the "Display login window as" area of the Login Options section of the Accounts preferences pane set to "Name and password" for this to work. And, obviously, you must have unchecked the "Show the Restart, Sleep, and Shut Down buttons" option in that same area.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050603213256255