Google Reader has now announced that it is now providing subscriber counts, the number of people who read your blog through feeds at Google Reader or the Google Personalized Homepage. In addition a new Google Reader Publishers Guide has been posted with tips on how to get add to Google buttons and answers to frequently asked questions such as how to view the stats.
They do so by adding a value to the HTTP user agent field when fetching your feed. In your raw logs, this may look like the following:
User-Agent: Feedfetcher-Google; (+http://www.google.com/feedfetcher.html; 4 subscribers; feed-id=1794595805790851116)