Today, Unity released a version of Unity 4.3 that builds to Xbox One. This functionality is only available to developers who have official Xbox One development kits from Microsoft (you can get one if you sign up for the ID@Xbox development program
Even developers who have as yet opted not to license Unity Pro will be able to publish to the Xbox One using Unity 4.3, as Microsoft will make a limited Unity Pro license available free-of-charge to ID@Xbox developers.
Notably, while some Xbox One-specific features have been integrated into the engine, developers will also be provided with plugins (and source code for those plugins) for Xbox One APIs. Also notable: No further versions of Unity 4.x will be made available for Xbox One, though the team will continue to update 4.3 to keep it functional for Xbox One development. The current version of Unity is 4.5.2, with 4.6 on the horizon; Unity 5 is slated to be released sometime in the future -- and it will be the next version of Unity that supports Xbox One publishing and development.
More details on this release are available at the Unity blog