In Mac OS X Tiger, the Login Window will now show most users, even if the UID is below 500, unless the account is UID 0 (root) or user whose default shell is set to /dev/null.
Tiger’s login window preferences now supports a new setting called HiddenUsersList, which is an array of one or more user’s short account name. To hide a list of accounts from the Login Window, run the following command:
# defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add account1 account2 account3 ...
where account1, account2, account3, … are the short user names for the accounts you want to hide. This would add additional names to any that might already exist.
To unhide all the names you may have previously hidden, run:
# defaults delete /Library/Preferences/com.apple.loginwindow HiddenUsersList
As always, make a backup of any preference file you might change so you can move back to it.