The Xbox One won't play existing Xbox 360 games, according to Xbox Live corporate VP Marc Whitten. The hardware architecture is too different, so you'll have to keep your current system plugged in to play the current generation of games.
"The system is based on a different core architecture, so back-compat doesn't really work from that perspective," Whitten told The Verge. This is due to the transition to x86 architecture, similar to the reason the PlayStation 4 won't run PlayStation 3 games natively. That puts both of the next-gen consoles in the same boat.