The most common I run into today is the router firewall. Most folk know about the ones in the Windows machines so that's the usual.
And yes, RDP did change with SP1 but it was not the culprit the last time I fixed one.
When I RDP to the machine, it will connect to my login screen. But when I type in my userID and password, it will come up with a certificate error message. It will ask whether i want to connect anyway. When I click yes, it will return to the IP address box and will not login. It used to work in the past. I read something about SP1 being the culprit.