IBM have shipped an early version of a Custom WCF Channel for MQ.  The dev team in Hursley contacted me to solicit feedback. It's apparently pretty simple now, supporting only SOAP one-way messaging, but they say if there is sufficient interest and feedback, they will consider developing it further and perhaps adding it to the MQ product (as they did with the MQ Classes for .NET, and other stuff).

Out of the box, WCF includes support for queued transport, over MSMQ, which is the Microsoft Message Queue, built-into Windows. This is available in the NetMsmqBinding. You can read more about it here.

What our friends at IBM are doing is similar, but different in a very important way: In the IBM channel, messages are encoded and sent via the message queue, just as with the NetMsmqBinding. However, with the IBM channel, of course, the queue is WebSphere MQ v6.0, and not the built-in MSMQ. There is one other important difference though: Messages are encoded using the SOAP over JMS (Java™ Message Service) message format described in WebSphere MQ 6.0. Why is this important? This enables interoperability with services and clients hosted by other environments that can also read or generate SOAP-over-JMS. Examples are such as CICS (Customer Information Control System) and WebSphere Application Server.  

So this channel has the possibility of opening up some new interop capabilities between .NET / WCF applications and apps that run in non-.NET environments, including Java applications running in WebSphere Application Server.

Please do have a look:
http://www.alphaworks.ibm.com/tech/mqwcf/