Took me a while to find this when I was looking for it so I'll reproduce it here.

If you want to make the little feed button light up in IE7 to indicate you have an RSS/Atom feed available then this is what you want to do;

For RSS add the following link to your page head
<link rel="alternate" type="application/rss+xml"
title="your feed title here" href="http://www.company.com/feedurl.rss" >

For Atom add the following link to your page head
<link rel="alternate" type="application/atom+xml" title="your feed title here" href= "http://www.company.com/feedurl.xml" >

I got this from the MS RSS Team Blog

It's got some useful stuff on it so worth a look.