I'm launching a product shortly that offers exactly this feature to Dropbox (or Google Drive, SkyDrive, ...). It's a native OSX and Windows app that keeps the keys on the users' computer and transparently encrypts and decrypts files before they're sent to your cloud storage provider.
The product is in beta right now but I'd love some more people to try it out. If you sign up at http://safeboxapp.com, you'll get a download link to try it out.