X does window forwarding, and I love it, but it doesn't do remote session resuming. For that, you have to fall to things like VNC, which really isn't much more than streaming video. I'm fairly certain that RDP doesn't do that. Its performance on pan-pacific connections is completely acceptable, much better than clever video streaming systems like gotomeeting or webex. I've never had the chance to try VNC across the ocean, but I'm guessing it wouldn't be as nice as RDP either.
Why do you think RDP does work by screenshot forwarding? I'm pretty certain it works by forwarding win API calls over a network, but I've never dug into the rdesktop source, so I could be completely mistaken.