I have studied the video and this is how I think he did it.
He used the same technology as the BBC use for weather forecasts – green or blue screens.
After watching the video (http://www.youtube.com/watch?v=kMIzR6GNAXw – use full screen mode) there are some give aways.
The cable from the tv set is not visible through the clear stand that holds the balls yet his trousers are visible when he walks behind the balls – suggests that the cable goes no where or more likely to pass through the green screen.
Derren is wearing all black which stops any “haze” caused by the green screen. In this instance I think he may have used a blue screen, look at the left hand side of the white base that holds the balls, note the blue tinge also look at the shiny reflection on his shoes – blue tinge.
Also watch Derren’s foot movements when he approaches the balls, he makes a lot of side step movements and sliding of feet – suggesting he is conscious of knocking or bumping into the screen behind him.
Finally note he didn’t guess the bonus ball – if he had there wouldn’t have been enough time to get all of the balls in place and set up.
Now I might be wrong, but this is possible and quite easy to do.