Looking at the Windows Runtime reference at the Microsoft site the only relevant collection I can find is the IObservableVector. Does the .NET Projection ObservableCollection get mapped to that WinRT interface? If so, hows does it handle the fact that ObservableCollection implements multiple other interfaces that the IObservableVector does not?
UPDATE: The ObservableCollection does work with Windows 8 Consumer Preview and can be bound to an ItemsSource and will correctly update the XAML control when items are added/removed from the ObservableCollection.
After asking on the Microsoft Windows Runtime Forum it seems that there is no mapping at the moment to the IObservableVector runtime type from any CLR type.