I've been doing a lot of work lately with StreamInsight, coming in SQL Server 2008 R2.
There are three development models you can use with StreamInsight: Implicit Server, Explicit Server and IObservable/IObserver.
When I was working through material on the IObservable/IObserver pattern, it wasn't immediately apparent to me where it had come from. It's based on the Rx Framework for .NET (Reactive Extensions). I finally got to watch the PDC Online session from Erik Meijer on the Rx Framework a few days ago and so many things suddenly fell into place for me.
If you have an interest in working with StreamInsight, I'd recommend watching Erik Meijer's session on the Reactive Extensions here: http://www.microsoftpdc.com.