What messaging platform would be the best choice for asynchronous message queuing (publish, consume) between PHP and Java apps? I have looked at RabbitMQ, ActiveMQ, OpenAMQ, Tibco Rendezvous, and Websphere MQ. Persistence is required. I love the idea of AMQP and using an open protocol, but I've heard of stability issues with RabbitMQ, which seems to be the leading AMQP implementation.

Messaging load will not be quite to "Twitter-style" scale, but will be fairly high volume and require very high availability.

An officially supported PHP API would definitely get points. Both Java and PHP systems will be actively producing and consuming.