Some good news recently that the Rx team have released the beta of Rx 2.0.
It is a move forward and will leave .NET 3.5, .NET4.0, Silverlight 3 and Silverlight 4.0 behind. For this release the supported platforms are .NET4.5 beta (+ Metro), Silverlight 5 and Windows Phone 7.1.
As the underlying platform has moved rather drastically there are some surface/api changes. So some old code may break. The good news is that the Rx team have worked hard to squeeze even more performance out of this release. Part of this is support for async/await pattern (which was in experimental releases already).
I am looking forward to giving it crack. Win8 seems faster, Visual Studio 11 seems faster and now Rx is faster. Happy days!