FAQ: Q54
Link: http://www.ilopia.com/FAQ/FAQ.aspx?id=54
Can I use a Windows 2000 License Server and CALs for Windows Server 2003 Terminal Servers?
No, you'll need a Windows Server 2003 machine to act as License Server. This is because Windows Server 2003 needs a new version of CALs. The Windows Server 2003 License Server can however serve both Windows 2000 Terminal Servers and Windows Server 2003 Terminal Servers.