I would assume that a "dumb" server like this could be load balanced behind any ol' load balancer. Anything from HAProxy to Amazon Elastic Load Blaancing. Each separate instance has no state, so you can spin them up as much as you want.

If you also do who-can-see-what permissions, then the database of that may need load balancing at some point (but that point is surprisingly far into the future.) Hash-based application level sharding will probably work just fine for this. Define 8192 logical databases, hash the user name, modulo by the number of databases, and talk to that particular logical database instance. As long as you don't database join across different customers, this will work just fine. (Or use Amazon SimpleDB :-)