Xbox 360 revisions may feature HD-DVD drive

While the first Xbox 360 consoles will use the current DVD format it is likely that future revisions may incorporate a HD-DVD drive. This feature will probably only be available for movies.

While the addition of a HD-DVD drive to Xbox 360 would allow future models to play HD movies stored on the format, it seems unlikely that the extra capacity would be available to game developers, since their titles would then not work on first-generation hardware.

More info at Gamesindustry