Microsoft's solution to building composite applications was the Prism project. In this new age of Windows 8 it was clear that Prism was far too overcomplicated and didn't fit the types of UIs that are used in Windows 8; it was designed for a radically different type of application style.
Today Microsoft announced the Kona project. It seems a lot more simple than Prism which is a good thing. My favourite MVVM library is Caliburn Micro that also included Windows 8 support but I'll definitely be checking out Kona to see if there are any new tricks for Windows 8.