cirulent, don't confuse Terminal Services and Remote Desktop Client with WebEx. WebEx is aimed towards presentation purposes, as for sharing one's desktop or workspace with a group of attendee's. Terminal Services is more a production environment, meant to distribute a workspace to single-user sessions.
Generally, this would be setup in an AD environment where a user would logon with their regular credentials as they would in the office. This wouldn't usually be a standalone box, where accounts reside on the local machine.
markdmac makes a good point about the web interface. However, that could also be misconstrued as a web application. In fact, the web interface is merely a "portal" for initiating the session and logging into the Terminal Server. After that, the Remote Desktop Client (RDP) is launched.
I hope you find this post helpful,
Jonathan Almquist
Minneapolis, MN