Thanks for your feedback.
Test transactions, submitted with the Silent Order POST method (with SSL), seem to be processed and recorded properly on the CyberSource Business Center database. Using the Business Center Search function, I could not detect any major anomalies.

The available CyberSource testing environment is not allowing all transaction types to be tested, but the hand shaking between ZenCart and CyberSource appears to be working as expected.

One transaction type I could not test was the case of a valid card number resulting in a declined authorization. There are 2 orders showing a 1 penny difference between the ZenCart Order Total and the Business Center Transaction Search Details (i.e. ZenCart $223.42, CyberSource $223.41).
I have an open ticket with CyberSource on these 2 anomalies.
Also, the CVV verification is not performed in test mode. I am planning to carefully read the CVS/CVV issues on thread=38848. Thanks Holgate on this one.

Thanks getyourgamehere. Your suggestion made updating the configuration.php files easy. Out of the 4 CyberSource payment Module files, only one had to be modified to accommodate SOP. I had to change one line in /included/modules/payment/cs.php to connect properly (around line 50-60): $this->form_action_url = 'https://orderpagetest.ic3.com/hop/ProcessOrder.do';
No modification to HOP.php.

With such positive test results so far, I am going forward with the CyberSource SOP Connection.

I spent one day testing with Authorize.net AIM but had a persistent problem (transaction cannot be processed). In any case, this subject should be covered in a different thread/post.

Thanks for your help,

deBeaujeu