We kinda started something... http://cloudHQ.net/dropbox Basically real-time synchronization between Google Docs and Dropbox (and other services). You can also synchronize Basecamp to Dropbox. Gmail and other Google and 37signals services are coming soon.

Backupify is also an option, but they backup to their own storage which, again, is not owned by you.

These guys look close:

http://www.backupify.com/