I'm adding this simply for future reference, so please ignore if you'd like.

You may have user accounts on your system (XP or Vista) that will never be used to login to your machine.  These may represent accounts created specifically for services or are simply logons for remote users to access otherwise secured resources.  It bugs me that through the Windows GUI you can't determine whether the account should show up on the Welcome Screen.  At least you can't do it intuitively; I hear you can do it through group membership.  Maybe I'll try creating a 'Hidden Users' group and adding users to it, removing them from the 'Users' group.  I don't know if that will work yet.

But you can remove a user from the Vista Welcome Screen simply do the following:

1. Open the registry (regedit.exe) to HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList, creating keys as necessary.  For example, I needed to create both the SpecialAccounts and the UserList key.
2. Create a DWORD value for each user you want to omit/remove from the Welcome Screen where the name of the value is the user name.
3. Assign it a value of '0'.
4. Reboot.

This same technique works in Windows XP, though I'd always used tools like TweakUi to accomplish this.  It's nice to know the manual way.