We just established that is a real issue with Authorize.Net. We were able to purchase a SHA1 certificate and we are now able to receive the Relay Response from Authorize.NET. Authorize.NET Relay Response does not handle G2/SHA256 certificates. This will become a major issue in 2014 when SHA1 certifictions will not be obtainable from vendors eg. GoDaddy etc.

I hope this helps someone.