Funnily enough, I just did a presentation on Monogame which, like XNA, is an abstraction for DirectX. And because the namespaces are shared, it's very possible to go between Windows 8, Windows Phone 8 from your existing code base in WP7 or Xbox. Checkout this link:

We can speculate all we want about XNA, Monogame has made great leaps forward in the 8-world, so my team and I are already working on porting our games.