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