All existing messaging solutions (WebSphereMQ, JMS, ...) use proprietary protocols. This is not a problem within a single organization. But between organizations, standard protocols are needed. Therefore, the B2B world uses protocols such as AS2, RNIF (RosettaNet) or good old (S)FTP(S).
AMQP is an initiative to bring a standard binary wire protocol to the messaging world. Just like POP3+SMTP allows you to retrieve and send emails using whatever email server, AMQP will allow any AMQP client to receive and send messages via any AMQP compliant server.
But when I read the spec, AMQP is focusing on the client-server protocol, contrary to SMTP that is (also) used for communication between mail servers. The AMQP spec states that a bridge should be used for server-2-server communication, but doesn't provide any details. As such, AMQP is focusing on messaging within the corporate firewall.
AMQP can be used for unbalanced B2B scenario's, where one side runs the AMQP broker. This is a setup similar to one big company or intermediary running an (S)FTP(S) server and smaller organizations putting and retrieving files from it. But for good decoupling, server-to-server communication is preferred. The server at the sending side will take care of delivering the message to the server at the opposite side. Like e.g. AS2 does: once an organization has an AS2 server in place, it becomes equal to all its AS2 counterparts.
With all this in mind, I was a bit puzzled by Paul Fremantle's enthusiasm about AMQP. In particular because he is the WS-RM spec lead.
WS-ReliableMessaging should have brought reliable async messaging to the WS-world. But it didn't. The WS-RM spec doesn't mention message persistence and so (most) vendors have an in-memory implementation, which is not reliable.
I still remember going through the book "Programming Indigo" and learning about the ReliableSessionEnabled binding property. What a disappointment to learn that for real reliability, one had to use the MsmqIntegration Binding and thus the proprietary MSMQ transport layer.