Membership.GetUser() returns null even though the logged in user is identifed (User.Identitiy.IsAuthenticated returns true)

I have been googling for the last few hours with no luck, any help will be greatly appreciated
Using VS2008, I implemented a windows authentication based membership using Scott's awesome post at http://weblogs.asp.net/scottgu/pages/Recipe_3A00_-Implementing-Role_2D00_Based-Security-with-ASP.NET-2.0-using-Windows-Authentication-and-SQL-Server.aspx
 Everything seems to be working fine, users are properly authenticated, role access works fine etc. I cannot, however, receive the user information with GetUser(). This returns null for some reason...
 I am making sure that I logged in by using User.Identity.IsAuthenticated and get true. Also User.Identity.Name returns the user name properly
I assume my connection string works fine as authentication works well
<add name="QualityConnectionString" connectionString="Data Source=tssloceng1\sqlexpress;Initial Catalog=Quality;Integrated Security=True;" providerName="System.Da ...