My colleague Rodney Viana has created a nice tool which simplifies troubleshooting of permission issues in SharePoint.

The tool tests various different settings in IIS and SharePoint and decrypts the permission masks involved when accessing a specific SharePoint Url.
A must have!

See here for details:
http://blogs.msdn.com/b/rodneyviana/archive/2010/11/11/application-to-check-cascade-access-rights-for-a-particular-user-or-anonymous-user-in-sharepoint.aspx?wa=wsignin1.0&CommentPosted=true#commentmessage