Mastercard and Discover changing their rules on debit cards
Authorize.Net has worked with MasterCard and Discover have worked together to extend these dates for all merchants using Authorize.Net solutions. Their merchants will have until June 30, 2011, to implement support for the requirements within their systems.
The steps that you will need to take to support the requirements depend on how you connect to the payment gateway. For example, if you connect using a shopping cart, point-of-sale device, or other solution, you will need to contact your solution provider to confirm that they will be supporting the requirements.
If you connect using a direct integration, you should contact your Web developer for assistance.
Authorize.net has more information here http://www.authorize.net/support/pafaqs/#do.