In case there's anyone who hasn't figured this out on their own, Google has updated their terms of service to make more explicit the fact that it scans Gmail for the purpose of matching email content with ads.
http://marketingland.com/google-tells-users-scans-email-microsoft-unscroogles-80150
In related news, Gmail turned 10 years old this past month, becoming the most popular email service just 2 years ago.
http://marketingland.com/happy-birthday-gmail-email-giant-turns-10-today-78630
Social tagging: email > Google > scan
