I had an issue with this module getting it to correctly interpret the response properly back from authorize.net. Authorize.net basically gave a shared account to me for testing purposes and could not modify any settings on their end. I ended up echo'ing the response from them back and it turned out that I was getting responses delimited using "|" and not ","
The card was being authorized, but this module was not interpretting the response correctly.
If you find this and change the line in authorizenet_aim.php, I got everything working correctly (ALL Test CCs passed, and the Test CC FAILED). I'm set to test Mode in the configuration, pointing to certification.authorize.net for "TEST".
This is around line 330 or so in the latest Aug 3, 08 release
// change this line to split using correct delimiter
// $response = split('\,', $authorize);
$response = split('\|', $authorize);