The reason this is occurring is the Grid.SortDescriptors.Reset() method is executing on the UI thread and hasn't finished by the time the GetRowForItem method is called. Here is a workaround that seems to work well for me. I'm calling the SortDescriptors.Reset() method, and then calling BeginInvoke (with DispatcherPriority of Input) to make the call to the GetRowForItem method.
private void GetRowForSelectedItem_Click(object sender, RoutedEventArgs e)
{
this.clubsGrid.SortDescriptors.Reset();
System.Windows.Threading.Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Input, new Action(() =>
{
var r = this.clubsGrid.GetRowForItem(this.clubsGrid.SelectedItem);
MessageBox.Show(r.ToString());
}));
}