gistfile1.cs

// This is the WPF/Silverlight ObservableCollection, not from Rx at all!
ObservableCollection<MyCoolModel> modelCollection;

model.Length
>>> 4

// Create a View Model Collection that will track the Model collection
var viewModelCollection = modelCollection.CreateDerivedCollection(
    x => new MyCoolViewModel(x));

viewModelCollection.Length
>>> 4

// Now, changes to modelCollection are mirrored in viewModelCollection
modelCollection.Add(new MyCoolModel());

viewModelCollection.Length
>>> 5