The numbers were printed on balls (like the lottery balls) but the balls were facing away from camera until after the draw was made when he then turned them around. He said that Camelot prevented him from revealing the numbers until after they'd been announced on the BBC (which i don't buy - there is definately another reason for not showing them beforehand).
I don't believe for a minute that he knew the numbers before they were drawn, but it appears that he did and its working out how he made it appear like he knew them that grabs people's attention.
Tim - I too saw 'The System' where he won on the horses and had read before that about the same thing with someone predicting the winners in each round, and eventually the final, of the FA Cup but 'predicting' results of a couple of hundred football matches or 50ish horses in 4 or 5 races is a bit different to 14million combinations in the lottery!!!
I personally don't think it'll be done using time delays or camera tricks as that isn't Derren Brown's style. Anything can be made to appear possible with technology these days, especially when viewed through a TV where we only see what the creator of the show wants us to see, but Derren is usually more sneaky than that.