Hi - I've seen too many threads asking about Authorize.net integration into a2billing. I have been using a2billing (kudos to Areski and rest of the team!) for over a year and while my implementation remains a hobby that still needs to see a profit, I thought I would share my working Authorize.net script with this community. It uses the AIM method (I think originally a2billing was using the SIM method provided by Authorize.net). Details on their APIs here:
http://developer.authorize.net/api/. They also provide sample code for various programming languages.
Refer to the attachment for the PHP script and follow the instructions there to replace a few parameters. Be sure to rename the file and remove the 'txt' extension when you save locally.
Good luck!