mongrel2 makes use of it in a very smart way as well. You don't need to tell mongrel2 about each and every backend process you're going to use. It's the other way around. The backend processes you start just connect to mongrel2 via ZeroMQ and bang they get their fair share of requests. The protocol is incredibly simple. You could implement it for a new language in an hour. The mongrel2 + ZeroMQ combination was really an eye opener for me in terms of how server-side processes should interact. This is going to be big.
Great project indeed.
I'm interested in using it for message passing and job queueing in particular from python using tornado.