I had a feed that was specified as a RDF+XML feed. Firefox’s autodiscover would find the RSS feed and put a little orange chicklet in the address bar. IE7 was not changing the color of its RSS chicklet when I visited the site so I surmised that it was not finding the feed. In conjunction with switching the feed over to FeedBurner, I changed my <link rel=alternate> statement from type=rdf+xml to rss+xml. Now IE7 correctly finds the feed.