In the last couple of days I've been researching Amazon S3 based filesystems, to figure out if maybe we could integrate that into an easy to use backup solution for TurnKey Linux appliances.

Note that S3 could only be a part of the solution. It wouldn't be a good idea to rely exclusively on S3 based automatic backups because of the problematic security architecture it creates. If an attacker compromises your server, he can easily compromise and subvert or destroy any S3 based automatic backups. That's bad news.