I thought Xamarin already released official support for async/await in PCL?
Xamarin have told me that they have not officially announced any PCL support (although their release notes show they have shipped bits).
MvvmCross ships using Profile104 because MvvmCross supports WP7 - but that doesn't mean your apps need to use Profile104 - you can use the profile you want to use (as long as it can consume 104)
Until Xamarin officially release their PCL support and more information about it, then I think the answer to this question remains at TPL on PCL of mvvmcross and Async/Await in portable class library targeting Windows Store, iOS and Droid