So far I've been leveragign the Device.Idiom functionality.
We're using MVVM light, so I have a common ViewModel and I've extended the INavigation Service so that i can configure it with both a Phone & Tablet page.
Then when I call
_navigationService.NavigateTo(PageKey, ViewModel);
The navgiation service will lookup the key in the configuration dictionary, and decide based on the idiom (Phone / Tablet) whether to the return the PhoneVersion or TabletVersion of the page.
http://developer.xamarin.com/guides/cross-platform/xamarin-forms/working-with/platform-specifics/