[gojomo]

OK, a typical website acting alone can't know what DNS servers my local machine is configured to contact, and furthermore doesn't use such DNS sensing to geo-localize its content (the claim I was responding to).

But, with the technique you've described, a website coordinating with a DNS server can probe to learn one of the DNS servers that gets consulted (directly or indirectly) by my machine. Got it. Neat and useful trick.