A post on Vgleaks points towards Xbox 720 requiring an "always on" connection, with an image taken from a Hardware Overview document pointing towards an "always connected" design feature.
It's also stated in the document that once games have been installed on the hard drive, playing from the optical drive "will not be supported." Boxed games will be used for distribution, but apparently during gameplay information won't be taken from the disc.
This alone doesn't confirm anything (and there's nothing to say that this document is accurate), but if proved true it does hint toward the possible end of the used-game market, with the new Xbox maybe set to employ activation codes as used on digital distribution sites such as Steam, making a disc redundant once it has been registered online.