This could be really useful one day and I love the idea of providing a REST api instead of a UI! Are you going to add support for european numbers (or would you open-source the code)?
the point is, that the pages are pre-rendered to static *.html files and can be served by a lighttpd or apache much faster and without any dependencies (ruby for example)
I'm wondering how you're going to encrypt the backups? Maybe you could do it in a way that you wouldn't be able to access the files yourself after they've been written? But yes, I'd definitely pay for it (~10$/mo).