Disclaimer: I am not a physicists, merely an interested layman.
Ok, when I saw this at first I didn't understand it at all. Using the traditional casimir effect you cannot produce energy. Nor could you use it to produce a force. Since the casimir effect involves two
conducting plates that push towards each other their net kinetic energy cancels out.
But the (not very informative) articles do mention something important: this proposal (whatever it is) is based on the (just recently confirmed) idea of a 'dynamic casimir effect'. I hadn't heard about that before so googled around a lot (especially using Google Scholar) and now suddenly it makes sense. It's, I dare say, almost obvious.
Here's a news article on the recent observation of this effect:http://phys.org/news/2011-11-scientists-vacuum.html
To quote a key passage:
"What happens during the experiment is that the “mirror” transfers some of its kinetic energy to virtual photons, which helps them to materialise."
So your not getting energy from nothing. You have to impart energy to the [virtual] mirror, and that energy gets transfered to the virtual photons making them real photons.
So how do you turn that into a method of propulsion? Easy. Just throw the photos outside the rear of your spacecraft.
How practical this would be would depend on a) how large you could scale up this dynamic casimir effect apparatus and b) how efficient the energy transfer in the effect is. In any case you still have to carry around a power source for the apparatus. I'm not sure this would really be any better than an ion drive.