Basically Google Chrome seems to be timing out on the ajax call:
But only if called from a secure page (which of course the checkout page SHOULD be).
Even when I fine-tune the Secure Pages settings to allow the line_items call NOT to be https, it still fails =[
So in my case, it seems to be the way Google Chrome handles ajax requests on a https page, but seemingly only when served by Cherokee (et. al.) and not Apache?
Very obscure... Still looking for a workable fix.