By default the login window retains the name of the last person to log in. This annoys me, since I usually have to erase someone else's name when I go to log in. To make the login window not display this information, do the following:
Edit the file /Library/Preferences/com.apple.loginwindow.plist. You'll need to be have root privileges, so you'll most likely want to open the terminal and sudo pico /Library/Preferences/com.apple.loginwindow.plist. Add the following two lines right after the <dict> tag:
<key>AlwaysClearUserField</key>When you log out, the user name field in the login window will be blank the next time you go to log in.
<true/>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011228074208750