>> From kotaku.com:
Microsoft reps said that any game planned for a Games On Demand will go through a "rigorous" QA process to make sure they work precisely as intended when played from a hard drive.

And mega blockbusters like Halo 3 and GTA IV may need to be "fixed" to become a Games On Demand, something Microsoft seems willing to do.

Oh and one other thing. Those Xbox 360 games you can buy as Games On Demand downloads will be available for purchase via the Web Marketplace, but not through Amazon.com.
Full Story: kotaku.com