I just submitted a suggestion on Microsoft Connect to move the INotifyCollectionChanged from System.Windows.dll to System.dll.
You can review it here: https://connect.microsoft.com/VisualStudio/feedback/details/560184/move-inotifycollectionchanged-from-system-windows-dll-to-system-dll
Here's the reason why I suggest that.
Actually I wanted to take advantages of the new feature of Silverlight/Visual Studio 2010 for sharing assemblies (see Go to the complete details ...