My favourite DB 'bit' was when he simultaneously played chess with nine of Britain's leading chess masters by using nothing more magical than his excellent memory. If I remember correctly (my memories not as good as his) he beat 6, lost 1 and drew 2, despite being a novice chess player.
He effectively played them against each other by copying their moves in all but one of the games (so scored exactly 50% against these players). He played one 'real' game against the only weak player in the group and managed to win (Derren is actually a reasonable - by most people's standards - chess player). I'm a strong chess player myself, have spoken to some of the guys on the show about this and they all of course knew what was going on after only a few moves. The really impressive bit was at the end of the show where he produced a pre-prepared envelope containing information about the positions on the boards at the end of the games, the opinion is about this is that an envelope swap must have been engineered at some stage, it is simply impossible to do!
I like the guy though, he is always entertaining imo!