using (SessionAwareCoreServiceClient client = new SessionAwareCoreServiceClient(binding))
            {
                RepositoryLocalObjectsFilterData filter = new RepositoryLocalObjectsFilterData();
                XElement checkedOutItemsXml = client.GetSystemWideListXml(filter);
                foreach (XElement tridionItem in checkedOutItemsXml.Nodes())
                {
                    String revDt = tridionItem.Attribute("CheckOutDate").Value; // null exception
            }