I'm pretty sure we kicked this idea around on my team at Expedia. They had a totally asynchronous, FSM design in C++ where program flow like:

class Foo {
int requestState;

void State1() {
//Enqueue some I/O operations
}
//Some time later this gets called
void State1Completed() {
//Use results of I/O
return transitionToState2;
}
}

There would be one Foo per request. It allowed for many thousands of transactions to be in flight at once, but it was miserable to code in; error handling was especially difficult.

Instead we wanted to build a system where there was one fiber (user-mode thread) per request and all I/O would be done with completion objects. That is:

String handleRequest() {
//Do some stuff
x = read(); //I/O operation, returns immediately
//Do more stuff
return x.getResult(); //Blocks
}

All I/O under the hood would be asynchronous and the event loop would swap in fibers in and out as they "blocked" on I/O. It is directly analogous to the FSM design. A fiber's stack is equivalent to the FSM "state" (class member variables) and handleRequest() embodies all of the member methods into one function.

Alas, I never had a chance to prototype it and now I work in Java which has no native support for fibers.