Disclaimer: This solution requires that you edit some code in the uc_authorizenet.module file. I know this is not the best practice, but it works.

See the "Authorize.Net Emulation" section at: http://www.eprocessingnetwork.com/Utilities.html

replace the line that has this: https://secure.authorize.net/gateway/transact.dll

with this: https://www.eProcessingNetwork.Com/cgi-bin/an/order.pl and it should work.

I am interested in learning how to make this into its own UC Module that acts as a standalone payment gateway. Will post here if I ever do it.

Hope that helps.

-Casey