I've been having a problem frequently in Leopard that I only occasionally had in Tiger, which is a login hanging when trying to logout after the Finder and Dock have closed. This leaves you with no apparent way to switch to other logged in users to at least log them out cleanly before restarting. Sometimes, the logout hangs before the Dock has gone, and it is a simple matter of clicking on the Finder icon to get your desktop back.
I found that a way of escaping from this trap is to simply employ the known hint of pressing the Option key plus a hardware key to go directly to the related System Preferences panel. This will summon up System Preferences and give you your menu bar back, from where you can then switch to another user account. Closing System Preferences will make the menu bar go away, but you can also bring it back again.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071226140954666