What is it
The version 5 branch of WCF RIA Services will be where cross-platform work will happen. My position is that supporting the same API across all platforms is more important than maintaining the current Silverlight API as-is. Accordingly, there may be breaking changes within the Silverlight client and/or server parts of WCF RIA Services to create a consistent API. If you want to contribute breaking changes to WCF RIA Services then the 5 branch is where you will want to be.
Multi-Client support (.NET/WinRT)
Support for all .NET platforms and WinRT. Includes Windows Forms, WPF, WinRT XAML, , MonoTouch, Mono for Android, and generic .NET
Planning on support BreezeJS