I work on RabbitMQ which is a messaging implementation that provides AMQP and other patterns. I hope my comment here can clear up some misunderstandings in the community.
Here is an introduction to AMQP from the RabbitMQ team - there are two presentations and a video - made at Google a few weeks ago: http://google-ukdev.blogspot.com/2008/09/rabbitmq-tech-talk-at-google-london.html
People who are wondering why AMQP is important compared to MQ, JMS, or whatever, should check out the first presentation. The second presentation includes information about XMPP vs AMQP. The two protocols are strongly complementary and are not alternatives. You can use these today: RabbitMQ as someone mentioned above has clients in lots of languages like Java, Ruby, Python,
Integration with Visual Studio has been done: http://www.rabbitmq.com/dotnet.html
As someone else pointed out there are protocol adaptors including STOMP and XMPP: http://www.lshift.net/blog/2008/07/01/rabbitmq-xmpp-gateway-released
It is open source - we welcome questions on the mailing list which you can find linked to from the home page: http://www.rabbitmq.com/