11/11/2012 update: Forgot about it, now Open Source, yay!
https://rx.codeplex.com/
https://github.com/Reactive-Extensions/Rx.NET
It also includes interactive extensions.
A quick search does not cause links to CodePlex, so I will say that this is not open source.
Bart De Smet has made many videos about Rx and how it works and how to use it:
http://channel9.msdn.com/Tags/reactive+extensions
To see the insides, if you know that the assembly is stored in it, the only remaining option is to parse it using something like ILSpy or dotPeek .
Will Microsoft open source? They released the .NET source some time ago, maybe they will do the same. This is not the same as open source, but you can at least enter the code and see what it is. I did not look, but the Rx material may already be there.
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