[toast0]

Have you given Erlang a look? What it calls processes sound like what you call fibers; although it does have quasi-premptive yielding as well as yielding when waiting for a message. Since Erlang is built on async message passing, you can easily send a request and block on the response, but you can also send multiple requests and process the responses as they arrive.

[sifoobar]

Sure, I played with Erlang back in the days. Preemptive kills the whole idea, which is why you can't share values. And it's probably the least integrated language I've come across.