I've spent the last couple of months continuing on an implementation of the System.Messaging library for Mono. I have since ditched the QPid client libraries (yes there are multiple, about 6 in fact) in favour of the single dll supplied by RabbitMQ messaging server. I am also using RabbitMQ as the main server for dev and testing. It great bit of software, really easy to install (on Ubuntu anyway) and interestingly built using Erlang.
Progress has gone really well, I have implemented all of the Receive, Send and Peek methods of MessageQueue including methods that select by Id/CorrelationId, use transactions and timeouts. MessageEnumerators are currently supported although it is not possible to implement the transactional version RemoveCurrent, due to the way the AMQP handles transactions. Well maybe not impossible, just an enormous, ineffient hack if I managed to get it to work.
The code has now be shifted into the mono trunk, which suggests I should really make sure I get it complete (or as complete as possible). One nice side effect of the implementation is that I have added an SPI that can be used by other potential implementations.
In similar news, a couple of weeks ago MS announced that they are joining the AMQP working group it will be interesting to see where they go with it.