If it's Win2K TS, no you don't. If it's Win2K3 TS, yes you do. One
thing to remember -- you get free TS Cals for as many XP Pro licenses
you owned at the time Win2K3 was officially released (April 23rd-ish,
I believe.)
And remember: even if the XP Pro clients get a free TS CAL if the TS
runs W2K, you still need to install and activate a Licensing Server,
since the free TS CALs are issued from the LS.