Hi Nick,
A simple test would be to find another PC/Mac somewhere and try that.
That's a great idea! I tried it on another 10.6.8 system and it also didn't show up but, just in case, I tried it on a system that happened to be running 10.7. No idea why but the USB modem appeared as expected! What's more, since that test it's shown up without a problem on my original system, too! Maybe Mac OS X "Lion" has heretofore unknown curative powers 
Thanks to all who replied - aside from the occasional "Serial port in use" that I can manage, I think I'm totally back in business!
Best regards,
-Patrick.