Accessing Terminal Services in general (whether you're using Citrix in addition to plain TS or not) on a W2k3 TS requires a W2k3 TS CAL per user/device accessing the TS. Citrix then has its own *additional* licensing.
Read this, it's the most concise guide on TS licensing available:
The Ultimate Guide to Windows 2003 Terminal Server Licensing
http://www.brianmadden.com/content/content.asp?ID=154