Yes, after I completed my testing in the PayPal Dev Sandbox, I entered the API credentials for my website and unchecked the Sandbox check-box in the Admin Gateway Settings. (See attached screenshot)
Once this was done, I was correctly directed to my PayPal checkout area and entered the payment information. Upon returning to the PS3 Confirmation page, there was no information in the areas where there should be and instrumenting the code indicated an authentication error.
The PayPal Tech Support person was able to monitor my site's communication with PayPal and indicated to me that while the initial Express Checkout API call was connecting to the correct PayPal "endpoint", the subsequent API call to transfer information back to the Confirmation page was connecting to an incorrect "endpoint", likely a sandbox.
Commenting out the code that directs API calls to the PayPal Sandbox endpoint as a result of a test of a variable, as I noted in my previous post, results in the correct connections between PS3 and PayPal, but of course disables future use of the PayPal Dev Sandbox for testing purposes.