Microsoft told developers to make their games without the assumption the HDD would be there. It\'s up to the developers how they use accessories; same as something like eyetoy or the HDD on the PS2.
I highly doubt HD-DVD games on the 360. The userbase would be so small, and it goes against the notion that Microsoft is trying to move everything over to being digitally distributed. They\'ve already announced HD Movies and TV shows being distributed over Live, and it\'s only a matter of time before full size games make it, too. If there are any HD-DVD games it\'s because the developer made that decision.