Mark Nottingham says Asynchrony: There Is No Spoon. To which I reply - "There is only FORK".
I really really like this approach - "A final approach would be to give the client its own explicit address (http://me.example.org/), modelling it as a resource as well. In effect, it becomes both a server and client at the same time. This pushes asynchrony to the application itself, by surfacing the different parties as different resources; this gives a lot of flexibility and power to application designers, but requires the client to be reachable by the server; a difficult task when it’s mobile or behind a firewall."
The 'behind a firewall' scenario needs somebody to put a custom Apache server on the client, build an Apache 'rendezvous' message-forwarding server and punch a hole out from the client to the server.
And what's this WARM stuff? Very interesting... must read...
"I’ve said before, HTTP can already provide reliability and high performance, and as we see here, asynchrony. What are the differences between SOAP and HTTP, then?" - go get 'em Mark!