An understanding of the risks and benefits of keeping your Cloud services in-house is crucial to any decision to use public or private (hosted) services.
The largest threat - believe it or not - is your own users and the effective management of applications, patches, and changes to your environment. You'll want to carefully determine the comparative costs of off-premises management, the security issues involvend, and your ability to monitor and manage internal (inside YOUR firewall) threats.
Thanks to John Kinsella and CSO
5 (more) key cloud security issues - CSO Online - Security and Risk