Microsoft as released Enterprise Library 4.0.

You can download it here:
http://www.codeplex.com/entlib

You can read about it here:
http://blogs.msdn.com/agile/archive/2008/05/16/enterprise-library-4-0-for-visual-studio-2008-released.aspx

You can read about the new features here:
http://msdn.microsoft.com/en-us/library/cc511712.aspx

You can read the online documentation here:
http://msdn.microsoft.com/en-us/library/cc512464.aspx