I recently ran into a situation wehre Windows 2003 SBS was refusing remote desktop connections completely because two people were logged in remotely. I had logged in to a customer’s Windows 2003 SBS to help troubleshoot various connectivity.  I needed to have the customer also be able to RDP to the server, but when they tried to connect to the server it said that it could not be reached.  This server was not running terminal services on it, which means that the server is limited to having two remote connections at a time.
Normally, on a regular Windows 2003 Server (not SBS), it will go ahead and allow the remote desktop connection to be established, but it will display an error message at login stating that the maximum number of sessions on the server has been reached.  In this case, it refused connections entirely for remote desktop, but the server could be pinged. What I didn’t realize initially was that there was another person logged into the server besides me.  When I logged off, then the customer could immediately contact the server again.