[billpg]

There are ISPs that will route all (or some) port 80 traffic into a bank of caching proxies but route traffic for all other ports normally.

(GRC.COM's 'Shields Up' service makes a point of using HTTPS to make sure it's testing your external IP rather than a random proxy server.)

I suppose it depends on what you actually want to use the returned IP for. As you say, the concept is hazy.