Passing user identity to Class Library

Hello,
In an ASP.NET application using a Class Library project, please tell:
1. How to pass user identity to Class Library ? (one way is explicit constructor/method parameter)
2. In default Form Authentication, how does ASP.NET take care of syncing Context.User and Thread.CurrentPrincipal ?
    http://www.asp.net/learn/security/tutorial-03-vb.aspx
    http://www.leastprivilege.com/CommentView.aspx?guid=d4405bb6-dec9-470e-ad9e-fcada13a4b0f
    http://weblogs.asp.net/scottgu/archive/2006/10/04/Tip_2F00_Trick_3A00_-Adding-Authorization-Rules-to-Business-a ...