Rx Framework is a .NET technology, so it won't be in WinRT as such (which is a native technology separate to .NET, and consumable by .NET).
However, given that Rx is produced by a team outside of the teams that produce Windows 8, Visual Studio and .NET, you would need to go and ask over on the Rx forums:
http://social.msdn.microsoft.com/Forums/en-US/rx/threads. We can't speak for their plans over here.
Base Class Library Team (BCL) | My Blog: http://davesbox.com
