XP used to include a terminal server CAL. It never included the regular server CAL's.
Now, there are two different types of Server CAL - the user CAL and the device CAL.
If you are using user CAL's, it doesn't matter how many copies of XP you have since it's tied to users, not the devices (generally, this is the preferred way). If you going to use device CAL, you would need a license for every device that would access any resource on the server.. technically, a device CAL is necessary if someone outside is accessing any internal/server resource (ie: a HOME PC that is using VPN, a client using a internet Kiosk to to check OWA, etc - or you end up shelling out the money for the external connector license).
note: There is also per server CAL, but that's really only very useful in very small environments.
If you have a select agreement with Microsoft, you can look at getting the core CAL which includes a windows CAL, exchange CAL, SMS CAL, and a Sharepoint Portal CAL. Obviously these are just the CAL's.. not the license for the software itself.. and yes, you need a seperate CAL for windows as well as the CAL for exchange (and SMS and Sharepoint Portal Server if you run it).
Gotta love Microsoft licensing.. if you need some help looking at Microsoft licensing, let me know - I can also put you in touch with a couple good resellers I trust.
