scottb ...
You have tried the following:
1 Made a transaction on AuthorizeNet's site to make sure the account is working
2 Removed and Installed Authorize Net AIM (make sure other payment modules are not installed and just disabled)
3 Regenerated your: Transaction Key
4 Cut and Paste from email your API Login ID and your Transaction Key
5 Checked that ports 80 and 443 are open both directions



