Doesn't have to be "brainwaves". The brain has a few outputs that can be highjacked (e.g. a computer with a neural interface that appears to be another muscle in the body). I don't know whether the bandwidth of these outputs is sufficient for interesting communication; we've evolved to take in far more data than we produce.

Edit It seems that more direct methods of neural interface are already plausible: http://www.technologyreview.com/biomedicine/37873/