Experience Smooth Streaming 1080p : The Official Microsoft IIS Site
They put up like a little benchmark thing you can run.
This is the tech that will be behind the new feature for the 360, where you can stream 1080p video as long as you have a 5 mb/s connection, without any buffering.
Works great for me, and the 1080p looks great.
Also, they said that you will still be able to stream videos without buffering, even if you don't have the 5 mb/s connection. They said that the video quality will automatically adjust to your connection speed.