Somewhere in the 10.4.1 or 10.4.2 update cycle, I noticed that all but two of the users on my computer disappeared from the Fast User Switching menu and the Login Window list. I think it was after I updated to 10.4.1, but I'm not sure (I didn't get around to trying to figure out the problem until today). Thinking that it was probably a permissions problem, I repaired permissions using Disk Utility, but that didn't fix it.
I finally discovered that the common characteristic between all of the accounts which had disappeared from the menus (but which were otherwise functional) was that I had set their shell to /usr/bin/false in NetInfo Manager. I tried changing one of the account's shell values back to /bin/bash, just to see what happened. Sure enough, when I went to the Login Window, that account had reappeared on the list. I subsequently changed the shell setting for the other accounts, and they reappeared as well.
So if anyone is tearing their hair out trying to figure out why a bunch of users have suddenly disappeared from the list of accounts, this might be your answer. I just hope this is a bug, because I liked the ability to set the shell to false for non-administrator accounts.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050721225946425