When developing WCF web services for SharePoint you may sometimes run into the following problem.
Security settings for this service require 'Anonymous' Authentication but it is not enabled for the IIS application that hosts this service.
Trust me, you will knock your head trying to solve this one. The solution is pretty simple though. In your web.config file modify your the bindings section with the following:
If you SharePoint application is using Kerberos authentication please change the mode to Windows.
This solution worked for me, hope that it will work for you.