Document : Licensing Windows Server 2008 Terminal Services

Microsoft licensing policies for Windows Server Terminal Services, including the new components that are included in Windows Server 2008, require that in addition to a Windows Server Client Access License (CAL), Microsoft® Core CAL Suite, or Microsoft Enterprise CAL Suite, you must acquire a Windows Server 2008 TS CAL for each user or device that directly or indirectly accesses the server software to interact with a remote graphical user interface (using the Windows Server 2008 Terminal Services functionality or other technology).

http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=b8a7c6ed-1bc1-4035-9110-1ee6da8f3fd4

http://blogs.technet.com/fabricem_blogs/archive/2009/04/13/document-licensing-windows-server-2008-terminal-services.aspx