Microsoft's Aaron Greenberg has confirmed that Ryse: Son of Rome will run natively at 900p, and the game will be upscaled to 1080p just like all other Xbox One games.
Here's the tweet
@deaconblade Update from team, Forza 5 is native @1080p, Ryse is native @900p, but Xbox One upscales all titles to 1080p & they look amazing
— Aaron Greenberg (@aarongreenberg) September 17, 2013
Ryse is scheduled to launch holiday season 2013 for the Xbox One.