W2K Pro clients do not need a WTS license. The W2K Pro license includes a CAL for Terminal Services. 9x/Me/Linux, etc. all require CAL's. All clients require an NT Server CAL as well including W2K Pro.
NT 3.x, 4.x clients require a WTS CAL.
Basically, everyone requires both an NT Server CAL and a WTS CAL with the exception being W2K Pro which only requires the NT Server CAL.