Sometimes the hardest problems have the easiest solutions...because the user is stupid. Yeah, thats me. Somehow I had saved an Aperture 3 trial serial number in a text file and was copying-and-pasting that instead of the serial number printed in my retail packaging.
The AppleCare rep Christian was a great help in discovering the problem. Correct serial number worked like a charm. Text file updated for future reference.
Lesson here: always check the details; make no asusmptions.
Thanks all, including Christian!