Using Feeds And Subscriber Data From Google
The Google Webmaster Central blog has a post "Tips on using feeds and information on subscriber counts in Reader." The post talks not just about the blog feeds, it talks about many sites, which have frequently updated content, resorting to feeds. If you have a feedreader for your site, you can get the number of Google Reader and Google Personalized Homepage subscribers.
We crawl feeds with the user-agent Feedfetcher-Google, so simply look for this user-agent in your logs to find the subscriber number. If multiple URLs point to the same feed, we may crawl each separately, so in this case, just count up the subscriber numbers listed for each unique feed-id.
You can also submit your feed as a Sitemap in webmaster tools. This informs Google about the URLs listed in the feed and heightens the chances of it getting crawled.
if you want to make sure your feed shows up in the list of available feeds for Google products, simply add a <link> tag with the feed URL to the section of your page. For instance:
<link rel="alternate" type="application/atom+xml" title="Your Feed Title" href="http://www.example.com/atom.xml" />