Remote encrypted backup solutions such as Carbonite rely on proprietary software both on the client and server side.

An open modular alternative sounds appealing. Providers such has geneticmail.com offer file hosting with webdav which should be a good and open backend.

!!! Modular approach with filesystem

One way to achieve remote encrypted backup with the encryption happening on the client is to:
* export remote storage as webdav
* mount the remote storage locally
* mount an ecryptfs filesystem on top of the local mount of webdav
* rsync the data from the local storage to the ecryptfs filesystem

This should be fine with Linux but requires Operating System and low level features. So it would be a problem on Windows. 

[{GraphViz title='Encrypted backup3' filter='dot' imap='false'

node [shape=box];

$(nodeAttributes)
"Local Storage"--"Backup Program"
"Backup Program"--"Ecryptfs view of medium"
"Remote storage"--"Remote webdav server"
"Remote webdav server"--"Webdav mount"
"Ecryptfs view of medium"--"Webdav mount"

}]