My Nov/Dec 2006 Internet Computing "Toward Integration" column just came out. This one is entitled "Advanced Message Queuing Protocol" (PDF), aka AMQP, and as its name implies it provides an overview of AMQP.
Over in the Apache Incubator, the Qpid project, an open-source implementation of AMQP, is up and running. As I mention in the column, you can find working Java and C++ AMQP brokers there, as well as client capabilities in Java, C++, Python, and Ruby. Qpid is where I spend almost all of my development cycles these days.
It's about time the industry had a standard, interoperable enterprise messaging protocol, and I think AMQP is on track to meet that need. If you're interested, get involved in Qpid.