Hiding users from the log-in window used to be a bit of a pain pre-Tiger. But that’s thankfully no longer the case.
In the bad old days, you’d have to change a user’s UID, which is fiddly process involving NetInfo manager and chmod. When Tiger came along it seemed to become even fiddlier; you also had to set the default shell to /dev/null, which isn’t really useful for hidden admin accounts that you actually want to use locally.
sudo defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add account1 account2 account3
Replace ‘account1 account2 account3′ with a list of accounts to hide. You will still be able to access them by clicking on the ‘other…’ pseudo-account and entering details manually. If you want to reset all accounts to unhidden, just miss off the bit at the end that starts “-array-add…”