We've patiently been awaiting a response from Microsoft on this issue and will update as soon as we receive one. In the meantime, we'd steer those Halo 3 latecomers toward a brick-and-mortar retailer.
Update: Microsoft's official response can be found after the break.
Microsoft's official response:
"Halo 3 has undergone extensive testing to ensure that players will have a positive experience, whether the game is played from a DVD, hard disk drive or USB drive. When installed to the Xbox 360 hard disk drive, players may experience longer than average mission and map load times, but regardless of where the game is installed, gameplay performance is not affected. Games on Demand delivers convenient, 24x7 access to a wide variety of featured Xbox 360 titles, letting people download games right from the comfort of their living room. Our goal is to ensure that Xbox LIVE Community has access to the best selection of games, and the freedom to choose how and when to access them."