Why Sqlite? It's really an awesome database, and using it in process from apache, we can achieve truly massive multi-tenancy, and really low cost.
We also have static file hosting, we're going for hosting of single page apps, but the database API is available over CORS for any domain.
Out of the gate our users get a subdomain on our site. But to serve webapps from a custom domain (ie www.mysite.com), or access the database API from CORS from a different domain, our tentative plan is to charge a small monthly fee (less than $10 a month) and remove the rate limit.
Yes, DDOS mostly, rate limiting is quite trivial.
We've built a tool called panic (https://github.com/gundb/panic-server) to test stuff like this, however correct we haven't finished integrating all the pieces.
Yeah, shoot me a message ( mark AT gunDB DOT io ) now, and I'll send you a ping when we're testing that stuff. :) Thanks for the feedback!