TS Licensing has got to be the biggest pain the ass to come along since... well... since Software Assurance.
It's hard enough to try and figure out the new licensing scheme without having to worry about unlimited pools, your date-of-purchase of XP client OS and just plain having CALs around for the server.
Along comes this site that I found today while searching around for more info. Called “The Definitive Q&A for Terminal Services Licensing” and it lives up to it's title. The author is the person who wrote the Terminal Services chapters in The Bible- Mastering Windows Server 2003 and so therefore is easy to read, as well.
It also has a link to a Microsoft site where you can punch in your product code for XP and get the sale/activation date and download a “freebie” grandfathered TSCAL token for your client machines.