Gonk:

MS has not said anything about HD-DVD support, and to be honest I wouldn't expect it from launch. If nothing else, the first gen units will not have an HD-DVD capable optical drive AND there will be no HDMI through the dongle adaptor on the initial units. Remember, HD-DVD has announced a delay, and to have it in Xbox 360 would be problematic.

I suspect you'll see it as an upgrade some time LATE next year.