The Xbox One will not feature any form of backwards compatibility for games.
This was just confirmed by Microsoft Xbox Live VP Marc Whitten in a comment to The Verge. "The system is based on a different core architecture, so back-compat doesn't really work from that perspective," he explained.
When asked if perhaps people would be able to download Xbox 360 games onto an Xbox One the way the PlayStation 4 will allow users to stream PS3 games, Whitten said no such plans were in the works.
On the plus side, Microsoft confirmed to Wired that your non-game media purchased over Xbox Live like movies and music will transfer over.