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 http://blogs.msdn.com/clrteam/archive/2009/12/01/sharing-silverlight-assemblies-with-net-apps.aspx). Everything went fine until I try to share a custom collection (with custom business logic) implementing INotifyCollectionChanged.
This modification has been made in the .NET Framework 4 (see https://connect.microsoft.com/VisualStudio/feedback/details/488607/move-inotifycollectionchanged-to-system-dll) so maybe it could be done in Silverlight too.
If you think this is justifiable you can vote for it.