Hi, I use VirtualQueryableCollectionViewwith following code (copied from here http://www.telerik.com/help/silverlight/using-data-virtualization.html
view.ItemsLoading += (s, e) =>
context.Load<Order_Detail>(query.Skip(e.StartIndex).Take(e.ItemCount)).Completed += (sender, args) =>
var lo = (LoadOperation)sender;
if (lo.TotalEntityCount != -1 && lo.TotalEntityCount != view.VirtualItemCount)
view.VirtualItemCount = lo.TotalEntityCount;
If TotalEntityCount was changed (new items were added on Server), VirtualItemCount is setted to new value. But setter of VirtualItemCount clears internal collection and fills it by null values. As a result view will contains empty placaholders for old records.