We do this all the time; it is how we support ouor clients remotely behind the PIX firewall without opening the ports for RDP to the world. I would first look at your post SP1 configuration and check the windows firewall rules and make sure that you are not blocking ip addresses that your vpn connection is making. the other thing to check is the DNS configuration on the PIX, if you can make the RDP connection from home, then you should be able to contact the remote server or desktop. also check to see if you are on the smae ip subnet that the remote network is on. are you getting specific event ID errors?