After installing .NET 3.0 on one of my test servers, I could no longer connect over RDP.  From the client, the connection would just terminate with no errors.  After looking at the Windows Event Log on the server, I found:

Event ID: 26
Application popup:  : \SystemRoot\System32\RDPDD.dll failed to load

After looking around, this appears to be an issue with the installed nVidia driver, though ATI users have experienced this as well.  Adding the following registry value and rebooting, I was able to RDP again.

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management]
"SessionImageSize"=dword:00000020