Woot! That's awesome!
I've found a limitation: on my machine, when I subscribe to a really busy topic like "#" (all topics) on test.mosquitto.org, the messagequeue buffer fills faster than Snap can process the queue. Eventually, the mqtt connection was closed (apparently due to some kind of error)... So, there's that...
Also, I selected "&&" as a delimiter between topic and payload for when the JavaScript function returns a message to Snap. I don't know if it's possible for an mqtt topic to contain '&&', so I don't know if this is a safe delimiter. I'll do some digging in this...
Mike