Just came across an interview with Brian McLaren on Next-Wave. It includes the following on defining "missional":
Next-Wave: Describe what it means to be missional.
McLaren: The term missional asks this question: what is the purpose of the church? To enfold and warehouse Christians for heaven, protecting them from damage and spoilage until they reach their destination? Or to recruit and train people to be transforming agents of the kingdom of God in our culture? The missional church understands itself to be blessed not to the exclusion of the world, but for the benefit of the world. It is a church that seeks to bring benefits to its nonadherents through its adherents.
"warehousing Christians for heaven" - conjures an image of an upmarket storage hire place on the edge of the city where Christians are preserved in a climate controlled atmosphere, almost dust free ... until the rapture.