Microsoft says that it's planning on bringing "entirely new original experiences to life" with the addition of Project Natal's controller free add-on later this year, not on reworking already existing games to make them Natal ready.
When Microsoft first showed off the controller-free Project Natal tech at E3 2009, it did so with both new content—Milo, Ricochet, Paint Party—and existing games—Burnout Paradise—with new motion-sensing control. Later, Natal-ready versions of Beautiful Katamari and Space Invaders Extreme were shown at Tokyo Game Show.
Microsoft's Aaron Greenberg told Kotaku at CES that we should expect more of the former, the new, built from the ground up Natal experience than traditional content repurposed a la Burnout Paradise.
"What we want to do with Natal is bring entirely new original experiences to life," Greenberg said. "These are going to be brand new original games and brand new entertainment experiences that never existed before. Because there's nothing else like it, developers are going to be making brand new content for us."
That said, Microsoft is open to all possibilities when it comes to Natal implementation.
"There may be some opportunities over time, where because you have millions of sensors in living rooms, that there may be elements of games that will integrate Natal into it," Greenberg added. It's unlikely that Microsoft would frown upon Natal integration in, say, the upcoming Xbox 360 game Halo: Reach, as developer Bungie has hinted at in previous interviews, later clarifying that it wouldn't do so until it "makes sense."