I bought a HD DVD player for my Xbox 360 the other day and promptly bought a HD DVD movie to play on it (with my 1080p TV!).
I was slightly confused as to why it was displaying at 1080i instead of 1080p and I just found out that it's because I am connecting with component cables which can only output HD-DVD to 1080i on my TV. Games are fine at 1080p (more than fine!!) but it looks like I will have to use a VGA cable to connect the 360 to the TV if I want to watch HD-DVD in the super fine 1080p setting.
I found out here
I can't wait to get back home and swap cables!