When the Security prefs pane is set up to require passwords upon wake, and multiple users are logged in, a user performing the "Switch User" function in the wake from sleep password dialog box experiences a faulty sleep event approximately 30 seconds after waking the computer -- even if logged fully into their account.
I should note if the user waking is the same user who was logged in when sleep occurs, then the 30 second sleep bug does not occur, hence most people will not see this issue. Only when the user who had the sleep event occur is different the user who incurs a wake event will this bug occur. Two workaround appear to exist.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031102202343870