MQTT protocol is an extremely light weight publish/subscribe messaging transport - targeted at machine to machine (mobile, industrial control, asset tracking etc) devices that came out of IBM in 1999. Its incorporated into Websphere MQ via the WebSphere MQ Telemetry product.
A key driver for MQTT is the explosion in connected devices - its predicted to be in the range of 50 billion by the year 2020. ActiveMQ already provides connectivity for asset tracking, smart devices and vehicle tracking via its ability to be deployed as a small footprint broker and utilizing store and forward for connectivity across unreliable networks. But it makes sense to extend the protocols ActiveMQ already supports (like OpenWire and STOMP) to support MQTT too (ActiveMQ Apollo already supports MQTT - and its the release where AMQP is being developed).
There's also an excellent open source broker called Mosquitto, that implements MQTT, but ActiveMQ gives you the advantage of interoperability with JMS,STOMP or C/C++/ clients over open-wire. In addition to providing enterprise features (clustering, networks, etc) - and of course - enables integration with Apache Camel (either embedded inside ActiveMQ or via the ActiveMQ Component).