I thought there was a client app that did most of the data management?Well, yes. But it's a command-line application, and all of the communication with my server is via a client-server API, not via a web browser -- maybe this isn't quite what you meant in your original question, but it certainly fits on the "without a UI" side of things.
if I had I desktop application and was a user of tarsnap could I bake backups into my app?
It would be difficult to bake tarsnap backups into your application directly, but tarsnap's command-line interface is almost exactly the same as tar's -- and I suspect that tar is one of the top 10 most commonly scripted UNIX programs. So if it would be very easy for your application to run tarsnap as a subprocess.
