Thanks to zooko for this one: A secure, distributed storage-service built on top of S3, which uses Tahoe (see previous article) as its client. Your data is 100% encrypted locally before being pumped into S3. It’s called S4.
It’s $25/month for infinite storage. For those of us with mountains of data to backup, it’s a deal (S3 costs about $30/T, currently, and even Glacier is $10/T).
Once you setup your Tahoe client with the right introducer and share configuration (which is trivial), all you do is call the “backup” subcommand with the path that you want to backup.
Done (well, after potentially several weeks of backing-up it will be).