Good explination of a feed for general information. I have but a few comments to add...
#1 The biggest + that was not mentioned is that this greatly cuts down on the spam problem. You subscribe to feeds and can un-subscribe by simply removing the feed. They do not have direct contact (email) information and therefore cannot spam you that way
#2 Yes, filtair, there are ways to tell when someone subscribes. If the rss / xml feed is served by a program (from a database) then you can just grab the ip address (in php:$_SERVER['REMOTE_ADDR']). After you have the referrers ip address just insert that into a file or db if it's unique along with a timestamp.
#3 How about a little practical application since this is a directory owners forum. I recently wrote a little tutorial about getting your directory fully indexed by search engines
. This was done a little while ago and works but I have updated my code a bit since, perhaps I'll head back and change it eventually.
This is related to this feed thread because I noticed the rss.php was my 2nd most popular page (behind index.php) and I had 0 links pointing to it. What happened is that many spiders visit that page 1st hoping to find a xml version of a site map. From there the spiders can visit the most updated pages, which is kind of the point of a feed, the freshest article goes first.
Hope this helps someone.
Viva la RSS ~ they will eventually replace email (thank God)