I've used rdiff-backup in the past for backing up my home directories to a friend's server over ADSL.  This works pretty well because rdiff-backup only ever sends the changes.  I can delete  all but the last 30 (or whatever) days of backups and it still always only sends increments.

I've recently started trying to use duplicity as I wanted my backups to be encrypted so that users on the remote server can't see my data.  I thought duplicity "builds on rdiff-backup and adds encryption" but I don't think it works the same.  As far as I can see, if I want to delete backups older than say 30 days in duplicity I need to have a full backup newer than 30 days.