If you'd like to put your Mac to sleep while sitting at the login screen, there's a way to do it ... at least, there is if you have an eject key on your keyboard. Control-eject will bring up a "restart, sleep, or shutdown" dialog box, even while logged out. So just log out and then hit control-eject, and you can put your machine to sleep.
It also works, of course, while logged in. I'm not sure of any workarounds for non-eject-key keyboards. Anyone?
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011011021428182