If I want to log out of one account and then switch to another, there is a delay while I wait for the current account to finish logging out and show me the login screen. There is an easy way to eliminate that delay with Fast User Switching (FUS).
When you choose 'Log Out' from the Apple menu a dialog pops up with a countdown. Ignore that and use FUS to switch to another account like normal. Once the countdown is complete, the first account will log out in the background by itself.
Note that this only works if nothing in the first account blocks the log out. So, if a document is open with some unsaved changes, a dialog sheet will ask to save changes and this will stop the log out process.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040328232433462