I'm reluctant to give up too much detail, but our infrastructure is very, very scalable and secure. It's pretty solid, and allows for very fine-grained control over many instances of any running application. Load balancing is taken care of, as well as redundancy.
Once we go public, it's possible we'll be more open about the architecture, but right now we're still iterating pretty rapidly, trying to make the user experience dead-simple without sacrificing stability or scalability.
That said, I'd love to join the beta.