IE Blog has a new article on how IE7 detects RSS feeds in webpages:
Well, feed auto-discovery works in a very simple way. Instead of trying guess the intent of the publisher and trying to figure out whether there's an little orange (or blue, or burnt sienna, for that matter) icon, IE will look for a specific "link" element in the HTML of the page that needs to be added by the publisher. We have published a short guide to what we're looking for here: http://blogs.msdn.com/rssteam/articles/PublishersGuide.aspx.
So, if a page doesn't have that "link" element, then IE7 won't detect the feed. Most sites that have RSS or Atom support include this element, but some haven't gotten around to it yet. So, if you find one, be sure to send them the link to the guide above.