A summary of why encryption wouldn't have prevented General Petraeus from getting caught. Like the quote: "If you think cryptography is the solution to your problem, you either don't understand cryptography or you don't understand your problem."
I got the General Petraeus story a bit wrong. He and his friend weren't using Google docs to communicate, they used a shared Gmail account. They would sometimes draft, but not send emails, as an extra measure of security, but, as this article points out, that doesn't work.