A significant amount of game code would have to be altered.
Microsoft has been using specially adapted versions of third party Xbox 360 games to help demo its upcoming motion technology Project Natal, but according to Microsoft's Kudo Tsunoda it's unlikely we'll see publishers patching existing titles to work with Natal.
Speaking to Destructoid at TGS last week, Kudo explained that existing titles would require a significant amount of game code to be altered in order to work with Natal. A game would have to be re-released with Natal support added.
Microsoft has been using EA's Burnout Paradise and more recently Xbox LIVE Arcade title Space Invaders Extreme and Namco Bandai's Beautiful Katamari to demo Natal.