That's really good advice - in general - if not in this case.
Via the standard Debian install, conf.d is empty:
#ls -l /etc/nginx/conf.d
total 0
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Conversely, downloading the squeeze package from nginx does indeed create a file
/etc/nginx/conf.d/default.conf which includes the following:
server {
listen 80;
server_name localhost;
which could have been the culprit - but wasn't in my original case (my first post does show an empty conf.d, and I've verified that on a separate machine).
But again, that's important advice you gave.
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Since, in my debugging, along the way I did remove the package from the debian repository and install the package from nginx.org, and so the /etc/nginx/conf.d/default.conf was on my system during the subsequent tests I documented above.
So, I now need to repeat the entire process and report back on it.
-S