Google exposed the number of Google Reader / iGoogle subscribers to a feed in the crawler's user-agent and in the Webmaster Console, but that's only useful for your own feed. What if you want to see how many subscribers Google Operating System has?
Now you can see the number of subscribers by searching for the feed in Google Reader: click on "Add subscription", enter the name of a blog or some parts of your feed's URL and you'll see the number of subscribers next to each (hopefully relevant) result. This number includes everyone who subscribed to a feed using Google, but the subscribers aren't necessarily active.
It's not very clear whether Google Reader shows the subscribers from all Google properties (iGoogle, orkut) or only Google Reader subscribers. FeedBurner's stats show higher numbers of Google subscribers than Google Reader.
Update: Mihai Parparita confirms that "these numbers include subscribers across all Google services". There's a difference between FeedBurner's numbers and Google Reader's numbers because FeedBurner counts the subscribers to all of the feeds that redirect to the FeedBurner feed.