I just took the time this afternoon and implemented a mqtt-gateway that works as mqtt-client:
it is based on the work of @Damme (http://forum.mysensors.org/topic/260/mysensors-mqtt-gateway) and makes use of nick o'learys PubSubClient-library (http://knolleary.net/arduino-client-for-mqtt/)
The main difference to the existing MyMQTT-gateway is that it will connect to an existing mqtt-broker (e.g. mosquitto). This is beneficial if you want to integrate with other mqtt-client-devices sharing a single broker for your automation solution. It also ensures your mqtt-client that is going to pick up the messages from the gateway will talk to a 'real' broker understanding the whole mqtt protocolls semantics
It works both ways:
on startup or reconnect it subscribes to "MyMQTT/#" (configurable as MQTT_PREFIX). Any publish-messages payload that is delivered by the broker to a topic that complies with the sheme <prefix>/<nodeId>/<childId>/<variable_type> is forwarded as a C_SET-msg to the MySensors radio.
Any C_SET-message received from a node is published to the mqtt-broker using the same sheme.