Just to come back to this:
I haven't exported using the latest release, but in r114 changing the following fixed the issue for me:
Browser.InvokeScript("if (window.C2WP8Notify) C2WP8Notify('activated');");
to
Browser.InvokeScript("eval", "if (window.C2WP8Notify) C2WP8Notify('activated');");
and
Browser.InvokeScript("if (window.C2WP8Notify) C2WP8Notify('deactivated');");
to
Browser.InvokeScript("eval", "if (window.C2WP8Notify) C2WP8Notify('deactivated');");
When we released a game on Windows Phone 7, you had to handle tombstoning, which was essentially saving the game state to isolated storage and loading it up again on resume. Looks like this is all done automagically on Windows Phone 8, yay.