The Windows Phone 8 and its SDK were presented these days.
This new OS has a shared core between Windows 8 and Windows Phone 8. Specifically the kernel, the network stack (IPV6, Bluetooth, NFC), file system, multimedia, graphics support (Direct 3D).
Microsoft invites the developers to use the C++. This mean the next apps will have more performances at games level. It results an incompatibility with the Windows Phones presently on the market.
In parallel, Microsoft announce the third party tools for all the developers.
For native programming, some editors propose render API, audio library or WP8 framework like Unity Technology, Hovok, Marmalade, Autodesk or Ogre.
The web programmer can take a look at PhoneGap, Appcelerator or Sencha and the C#/XMAL programmer at MVVM Light Toolkit from GalaSoft, RadControls, XMAL Spy or Xamarin Mobile.
What is mean for XNA developer?
It will NOT possible to create a new project targetting WP8
However, the apps developed for WP7.x will be supported by WP8
More details under this link: http://msdn.microsoft.com/en-US/library/windowsphone/develop/jj207003(v=vs.105).aspx
It seems the future of XNA is not so lighted.
Download Windows Phone 8 SDK