The basic principle is fairly straightforward: pairs of speakers generate a standing wave (at ultrasonic frequencies) at a focus point, and the motion of the air molecules is enough to keep the particles suspended. With some clever engineering, by varying the phase of the sounds, the can move the focus point (and also the particles). Sadly, it's had to see how this could be scaled up to macro-sized objects though.
That's all for this week! See you back here on the blog on Monday.