I use VNC to do remote support (or Screen Sharing under Leopard). If you do a lot of remote support, there are times when you leave computers idle while you deal with other things. You need the owner/user of the computer to leave it alone, so as not to undo what you are doing. For example, when restoring data from a remote backup.
Typically I will put up a Stickies note on the screen warning the user that I am still working. But what do you do if the computer must be logged-out in order to do your work?
The remedy is simple. First, log out on the remote machine. If the login window then displays a user name and blank password field, press Escape to get out of the password prompt (so that you are looking at the list of users). Then hit the down-arrow to highlight the first user, and press Option-Return to get to the Name and Password prompt. However, instead of entering the username, type "Still Working!" (or whatever you prefer), and you're reasonably safe from user intervention (at least from reasonable users).
Of course, if the remote Mac's login window comes up with the user name and password fields, you can just type "Working..." (or whatever you prefer) in the username field.
[robg adds: An older hint here stated that 10.4 users had to use Shift-Option-Retun to flip between the two login window styles; in 10.5, Option-Return appears to again work as it used to.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080307070647136