Google rolled out a great new security service for Google Apps Unlimited domains. Gmail now provides Data Loss Prevention (DLP) services for domain administrators.
Gmail DLP allows you to scan email traffic for your organization for content such as credit card or social security numbers, and set up policy-based actions when this content is detected. Available actions include quarantine, email rejection, and modification. Gmail's DLP covers common attachment types as well!
Google has also announced they're working to bring this functionality to Google Drive.
Check out the DLP whitepaper for more information.