Fun fact of the day, Google is kind enough to include the number of subscribed readers each time their feed fetcher polls your feeds.
18.104.22.168 - - [10/Jun/2010:22:37:47 +0200] "GET /feed/ HTTP/1.1" 304 - "-" "Feedfetcher-Google; (+http://www.google.com/feedfetcher.html; 5 subscribers; feed-id=3719683273676272306)"
That makes it easy to check how many subscribers you have to a certain feed.
The feed-id is a 64 bit identifier for the URL of the feed, so if your feed can be accessed via multiple URLs you might see multiple values in you logs. Go sum them up!