I think Derren Brown uses this as a mind hack a lot.: http://www.youtube.com/watch?v=3Vz_YTNLn6w (notice specific diversion into spatial memory, it's probably been tried and tested as the best distraction from the color of money in hand)

I feel that mental focus if VERY weak and very exploitable.

As a side note, I think there is another, less obious, mental hack going on, on the audience. Derren claims (in the intro to this TV series) that there is no acting here, but a lot of misdirection. I believe it. I think when he shows this trick work 2 out of 3 times, it's probably more like 2 out of 30. My guess is that he biases the sample quite cleverly, showing 3 cases is exactly the minimum that you can show giving the impression that a) reporting is honest (see - I showed a failure!) and b) the 'magic' works in most cases. Also I think getting caught/embarrassed by a hot dog vendor evokes certain associations that yeah, he can be beat which prevent you from thinking how much he can be beat.

Here is to you Derren, Master of Dark Arts.