I'm as guilty as the next church planter/armchair theologian at throwing around the term "missional."
Here's a smart guy (Michael Frost, co-author of the very good Shaping of Things to Come) helping to cut through the fog that is billowing around the term these days. He talks a lot about this stuff over at Facing the Challenge.
Want more? Go read Missional Church. Still probably the best treatise on the theology and thinking around the subject.