Multi-threaded servers are indistinguishable from single-threaded servers from the client's point of view. In fact, the designer of a server can just "turn on" multi-threading by starting another thread.
In any event, the server can still make use of multiple CPUs in an SMP configuration, even if it is servicing only one "client." What does that mean? Let's revisit the fractal graphics example. When a subserver gets a request from the server to "compute," there's absolutely nothing stopping the subserver from starting up multiple threads on multiple CPUs to service the one request. In fact, to make the application scale better across networks that have some SMP boxes and some single-CPU boxes, the server and subserver can initially exchange a message whereby the subserver tells the server how many CPUs it has — this lets it know how many requests it can service simultaneously. The server would then queue up more requests for SMP boxes, allowing the SMP boxes to do more work than single-CPU boxes.