Eidos president Ian Livingstone stated in his answer to the question of the possibility of a digital-only gaming landscape that the next XBOX may use different CD's. These CD's would be watermarked so that they won't work on another console.
Here is the full quote:
“With the next Xbox, you supposedly have to have an internet connection, and the discs are watermarked, whereby once played on one console it won’t play on another," Livingston said. "So I think the generation after that will be digital-only.”