Microsoft Knowledge Base Article 823313 contains the following summary:

This article contains information that can help you understand and successfully deploy Terminal Services on computers that are running Microsoft Windows Server 2003. This article discusses the following topics:

Licensing requirements for computers that access a Windows Server 2003-based terminal server (a server that has Terminal Server enabled)
Transitional client access licenses (CALs) for Microsoft Windows XP Professional-based client computers
How to deploy Windows Server 2003 Terminal Services
Management Limitations for TS User CALs
Information about how to determine whether to contact Microsoft Product Support Services (PSS) or Microsoft Clearinghouse

For additional information about Windows Server 2003 Terminal Services licensing, visit the following Microsoft Web site:

http://www.microsoft.com/windowsserver2003/techinfo/overview/quickstart.mspx