Steve Riley gave this presentation on security in Amazon Web Services back in 2010. Here's the abstract:
Moving to the cloud raises lots of questions, mostly about security. Amazon Web Services has built an infrastructure and established processes to mitigate common vulnerabilities and offer a safe compute and storage environment. In this session, we'll discuss common cloud security concerns, show how AWS protects its infrastructure from internal and external attack, and explain how you can take advantage of the security features of AWS in your own applications as you extend your enterprise into the cloud.