A 2003 TS Licensing server can handle both W2K and 2003 TS CALs.
If you install 2003 TS CALs, you can use them to access a W2K TS,
but only as Per Device Ts CALs.
From Windows Server 2003 Pricing and Licensing FAQ
http://www.microsoft.com/windowsserver2003/howtobuy/licensing/pric
licfaq.mspx
<quote>
Q. Can I downgrade a TS CAL for use on a Windows 2000 terminal
server?
A. Yes. You may downgrade Windows Server 2003 TS CALs. However,
both TS User and Device CALs will act as device-based CALs when
accessing a Windows 2000 terminal server, as TS User CALs are not
supported.
Q. How do I designate Windows Server 2003 TS User or Device CALs
or Windows 2000 TS CALs?
A. During set-up, the Terminal Server Licensing Management (TSLM)
will prompt you to choose.
</quote>