Yesterday, Google updated its terms of service. In so doing, Google has revealed that they in fact scan outgoing and incoming e-mails so that they can send targeted ads to you:
Our automated systems analyze your content (including emails) to provide you personally relevant product features, such as customized search results, tailored advertising, and spam and malware detection. This analysis occurs as the content is sent, received, and when it is stored.
Users of Google's services—just about everybody?— are encouraged to read and understand the new terms of service.