title: Is the Reactive framework available for .NET Framework 3.5?
body: <p>IObservable and IObserver would be quite handy right now for something I'm working on.</p>
  
  <p>Is there a similar set of interfaces and classes available for .NET 3.5?</p>
  
  <p>Or must I wait for .NET 4.0 before I can use them?</p>