You could use also Apollo, ActiveMQ's successor, which comes with an MQTT adapter and runs on the JVM. Has the added benefit of supporting other messaging protocols as well. I'm not sure how well mosquitto would hold up, do you have any experience with it?
As an aside, MQTT has a constrained device optimized sibling MQTT-SN, which works over UDP and supports sleepy nodes, etc. Unfortunately that one's stuck in licensing hell and does not yet have good tooling, at least last time I checked.
My company selling Telecom-grade distributed MQTT broker and MQTT-SN gateway. Both written in Erlang/OTP and come with 1M concurrent clients SLA (was tested with even more).
I gave a talk about MQTT-SN Gateway at Erlang User Conference 2013:
As an aside, MQTT has a constrained device optimized sibling MQTT-SN, which works over UDP and supports sleepy nodes, etc. Unfortunately that one's stuck in licensing hell and does not yet have good tooling, at least last time I checked.