On my home computer, I usually have to switch from one of my childrens' accounts to mine, as they forget to log off. I have found a way to log off their accounts, and log onto mine at around the same speed it would take to just use Fast User Switching.
Firstly go to the Log Off Dialog box (I usually use Shift-Command-Q, but you can go to the Apple menu and click Log Off). Next go to the Fast User Switching drop-down menu, and select the user you want to login as. Type in the password for the user and press Enter. It takes one or two seconds to switch, so during that time, click the Log Off button in the dialog box. Even if you don't have time to press Log Off, the user will log off after 120 seconds automatically even on the new user.
There you go, you have logged off the old user and logged on to the new user just as fast as using Fast User Switching.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006082303234812