I would like to see the option to support encrypting the backups with a passphrase prior to uploading them offsite, perhaps using gnupg to do so. (gpg -c)
How difficult would that be to implement? The only difficulty I can see would be in the restoring phase. It might require twice the disk space since you would have to download the encrypted backup, and then decrypt it, which would leave 2 copies temporarily on disk.
It would be a very nice feature to have, however.