Cycles: How can I make a plane both emmissive and transparent? I have the following situation:

I have a scene lighted by a "sun" lamp that gives me good sharp shadows. But I also want to have a large plane with an emissive material placed above the scene to regulate the "general" brightness. My problem is that while this plane does emit light the way I want it to, it also blocks the sun and casts a shadow on my scene. So I want it to both emit light, and by invisible to the rays of the sun. How can I achieve this?
Go in the Properties panel > Object > Cycles Settings, and disable Camera and Shadow. And if you don't want to see it in Solid mode either, go in the Properties panel > Object > Display > Maximum Draw Type > Wire.