Nanomsg, a fork of the ZeroMQ broker, is moving along smoothly this year. It recently reached version 0.3.3, which I didn't find much information on, but I thought I'd get you caught up if you're not aware of this interesting project.
First off, you should read ZeroMQ-creator Martin Sustrik's blog – "Why should I have written ZeroMQ in C, not C++" Part I and Part II.
Then check out Callback Hell, also written by Martin Sustrik.
Finally, here is the coverage of Sustrik's new project "nanomsg," which is his re-envisioning of a ZeroMQ-like broker.