RabbitMQ is an implementation of AMQP, the emerging open source standard for high performance enterprise messaging. Think of AMQP as the open source version of something like MQ Series or other high-volume JMS servers.XMPP is open XML technology for presence and real-time communication.
Message volume shouldn't be a. RabbitMQ with an XMPP gateway might be part of the the solution. The combination means you can inject very high volumes of messages into IM or other XMPP enabled applications with far fewer scale issues.
For the geeks:
The mod_rabbitmq module implements an ejabberd extension module which gateways AMQP (as implemented by RabbitMQ) to XMPP.
Via James Governor Monkchips on Twitter.