XNA is already available for windows 8.x via the open source MonoGame project. In fact MonoGame has had support for Windows 8 for over a year now. So if you like the XNA api, it lives on in the OSS world.
I would like to add information on MonoGame is available at
site : http://www.monogame.net
or Follow us on twitter for the latest news and updates
twitter : @MonoGameTeam
And hang out on IRC to ask questions.
irc : irc.gnome.org #monogame
Just so you know, we recently added the ability to reuse your Game class within a Xaml Based Windows 8 applicaiton. see https://github.com/mono/MonoGame/wiki/Windows-8-Project-Types for details