We just released the second beta drop of the 'ArcGIS Runtime for .NET SDK', which now supports both Windows Store 8.1, Windows Phone 8.1 in addition to WPF. This means you can now build universal apps with a map control that supports all these platforms. We also released the package on NuGet for your convenience.
As a demonstration how quick it is to create a Windows Phone and Windows Store app, here's a little video creating a universal map app from scratch in 80 seconds:
Note: The nuget package currently doesn't support WPF. You will need to download the full setup to get WPF support as well, instead of using the nuget version.