Microsoft Japan's Yoshihiro Maruyama has said that an HD-DVD Xbox 360 add-on is possible, "but it won't have any relationship to gaming." Say what? Maruyama has stated (via Xbox Famitsu) that if the HD-DVD format was introduced onto the Xbox 360, it would only be used for viewing movies that are produced in that format—games will remain in standard DVD format.
If true, it's likely that Microsoft does not want to confuse consumers with yet another hardware configuration (alongside the premium and core setups). Microsoft is dedicated to its stance that (almost) every Xbox 360 game will play on the core system. But why not allow publishers to release games in both formats?