Here is a whitepaper from InformationWeek on cloud security that is worth reading. Follow the link at the bottom to download.
Here is the overview: "A common question about the cloud is whether it’s more or less secure than a corporate data center. That’s an interesting question—and also the wrong one to ask. Instead, customers and potential customers of cloud services, whether IaaS, PaaS, SaaS or some other acronym, need to ask whether a cloud provider’s controls align with the amount of risk a customer is willing to take with its data."