I have upgraded my Vista to Windows 7 RC but can no longer use the Remote Desktop Connection in Accessories to connect to the Desktop of a server running Windows 2008. The message is that there is no license available for the client. This was working under Vista.
Before installing Win 7 I installed Win Server 2008 to see if it would work as a workstation (way too much armor <G>). I noticed the remote desktop server required client licenses to be installed within 30 or 60 days. Maybe that is your issue -- a limitation on the server end? Just a guess but Vista probably had the credentials it was looking for and Win 7 doesn't.