So i switch to NAX about a month ago and start adjusting some of my WP apps to use it as their SDK,
For the apps that i show banners everything is technically ok but i have an app that i used a full ad interstitial and the app got rejected in the WP Marketplace due to the reason that you cant use the back button to navigate back from the ad interstitial page (and they are correct, you cant and its against the Marketplace guidelines) you can only use the X button to close the full page ad (are Ad pages exempt from this? i'm not aware of it).
I dont know if this is the case also with the WP8 SDK but i suspect that it could be.
(I've used the WP7 sdk because its a WP7 project which uses xna+sl and doesnt benefit from any of the WP8 features, so no point in converting it)