Well, I'll admit that grad student salaries don't put me in front of plasma TVs that often, so I don't know that much about them, but here's a quote from the article:
So Agile Mirror is made of plasma (maybe the operational parts of plasma TVs are too - I don't know), and it's the plasma that is interacting with the microwave radar - reflecting it, I think. You've said that it does this by varying the opacity. Ok. How? What's the physics behind making a variable-opacity sometimes-reflective sheet of plasma?