I've been trying to find more details myself. It sounds like the ability to develop apps is coming:
http://arstechnica.com/gaming/2013/10/wait-can-the-xbox-one-really-run-generalized-windows-8-apps/
Nothing to announce today, but when I talk about that common core,
you're seeing that common core in action, being able to target other
devices over time that run the Windows 8 engine.
If you want to know about how to get a head start about thinking about
developing for Xbox One, the logical thing to do is [to] go build
Windows 8 applications.
There are some additions in WinRT designed to allow interaction with Kinect:
http://www.oxm.co.uk/59024/features/what-its-actually-like-to-develop-for-xbox-one-microsoft-explains/?page=2
The applications are very similar to Windows 8, like we brought a lot
of the Windows 8 namespace - WinRT can be used for building those
applications, the time it takes to port a Windows 8 application to the
new Xbox? It's super, super easy. There's some things we added to the
platform for handling gestures, for handling voice and handling ten
foot UI paradigms.
Right now it appears the most you can do is try out your app ideas in WinRT and wait until an Xbox One SDK appears.