TLS authentication overview
Remote Desktop Protocol (RDP) provides data encryption, but it does not provide authentication to verify the identity of a terminal server. In Windows Server 2003 Service Pack 1 (SP1), you can enhance the security of Terminal Server by configuring Terminal Services connections to use Transport Layer Security (TLS) 1.0 for server authentication, and to encrypt terminal server communications....

Prerequisites for Configuring Server Authentication
By default, Terminal Server uses native RDP encryption and does not authenticate the server. For TLS to be used for server authentication and encryption of terminal server communications, the server and client must be correctly configured.

More information :
http://www.microsoft.com/technet/prodtechnol/windowsserver2003/library/ServerHelp/a92d8eb9-f53d-4e86-ac9b-29fd6146977b.mspx