Indeed, but if Derren Brown guesses your mobile number, it's probably a "trick" rather than "mentalism". ThisSpaceAvailable has claimed that he can manipulate people. I would argue that this is weakly true, and he uses it for the simpler tricks he performs, but for the really impressive effects he probably falls on traditional magic tricks most of the time. The card trick by Simon Singh demonstrates that: he hasn't used mind manipulation to pick the cards, he's used a standard card trick and dressed it with the language of "mentalism".

Note that I make no claim that there is anything wrong with all this! But Derren Brown is trying to fool you, and that is to be remembered. He also does a similar thing to Penn and Teller, where he shows you how some of the trick is done but leaves the most "amazing" part hidden (I'm thinking of the horse racing episode, which was great, and the chess playing trick)