I'd like to offer a service where customers can write arbitrary java code, send it to me, and I'll run it for them on Amazon EC2. My question is: how can I do this without exposing one customer's data to another customer?

Right now I'm thinking that each customer can be sandboxed as their own OS-level user with restricted permissions. Is that good enough?

I understand that this is a tricky issue, but it seems to be one that many people, such as the designers of multi-user OS's and Amazon themselves are solving, so I am optimistic that there might be a good approach.