Trouble adding WCF service reference using ObservableCollections

Question

I have a silverlight 3 project that references some WCF services in the hosting web application. Previously, when I added the service reference, I had the option on the Advanced tab to generate the message contract using System.Collections.ObjectModel.ObservableCollection. I just upgraded to Windows 7, and am working with the project again. Now, when I try to add the service reference, the ObservableCollection option is not available for the "Collection type:" option, just System.Collections.ObjectModel.Collection.

Does anyone know how to get the ObservableCollection option back?

Thanks for your help.