If you upgrade from 10.2 to 10.3, you may find that the Shift-Tab key combo for switching backwards between fields doesn't work properly -- this can be rectified by removing the .GlobalPreferences.plist file from both /Library -> Preferences and ~/Library -> Preferences.
You will need to Log-out and back in again for it to have any effect.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031103095110513