As much as I hope this is true source a from Dutch Microsoft employee,
"Microsoft said that they would only use the HD-DVD play for movies, but it seems like they're also gonna use it for their games. They are gonna pack a normal gamedisc (like we know it now) with a second disc that puts the same game on a HD DVD. This disc contains better quality movies and extra content for the game, content that gamers without a HD-DVD player can download from Xbox Live Marketplace as well, for free! This all was said by a Dutch Microsoft employee on a event for retailers."
Mind you this would be amazing if it would be true. I just can really read into Packing HD-game with Normal disc with out rasing the price of the game.
We can only hope that this is true.
***Update***
CJPC did call this about two weeks ago, for the record.