I'm running VS2010 RTM w/WCF RIA Services V1.0 for VS2010 and Silverlight V4.
What I found was that the CategoryDomainDataSource.HasChanges = true which causes CanLoad to be false.
Changed the code to:
private void CancelButtonClick(object sender, RoutedEventArgs e)
Now it works. Is there a better way to handle this? thanks!