This one shouldn't come as much of a surprise, and Sony have said the same thing about the PS4. Essentially, due to the x86 architecture employed by the Xbox One, the new console "won't play any existing Xbox 360 games".
The news came via Polygon, who caught Microsoft's corporate vice president of Xbox Live, Marc Whitten, for a quick interview.
The system is based on a different core architecture, so back-compat doesn't really work from that perspective," said Whitten.
Although there's no concrete confirmation that this will hold true for smaller XBLA digital titles as well as the full retail releases, Polygon stressed that Whitten had informed them that "the Xbox One won't play any existing Xbox 360 games".
It remains to be seen whether or not Microsoft will attempt to provide a cloud-based retrospective solution as Sony have hinted at with Gaikai.