Sounds like a great service! Data Locker:
The explosive growth of personal cloud services like Dropbox sends a clear message that computer, tablet, and smartphone users greatly value synchronized data access across all of their devices. But many users struggle with the trade-offs between convenience and concern over the security of personal data stored in the cloud. DataLocker eliminates this concern by enabling users to easily encrypt sensitive information in their Dropbox account without sacrificing the convenience of anywhere access.
The DataLocker suite, offered for free by AppSense Labs, includes native clients for Windows, Mac, iPhone, and iPad. The Windows and Mac clients are capable of encrypting and storing secure files in any local file system or cloud storage location, including Dropbox. The DataLocker iOS application stores files in Dropbox. (Additional storage options are planned in future versions of the iOS application.)