Via Stowe, from Google Now Reporting Subscribers:
“Starting with our Saturday February 17th subscriber reporting, FeedBurner publishers will be able to see how many Google Reader and Google Personalized Homepage subscribers they have. (Thanks, Google!) This information will show up in tonight’s subscriber reports (meaning that most of you will start to see the data on Saturday morning, U.S. Central Time).
What does this mean? This is one more data point to help you understand how many people have asked to receive your feed (aka “subscribers”). For those who are interested in the particulars, the number that Google is reporting is the total number of Google users who’ve subscribed to your feed in Reader or with Personalized Homepage.”
So there ya go.