How about using the 1st section of Kent Beck’s Money example. It starts out very simply but when you arrive at addition of two different currencies, TDD suddenly shows you the falisity of up front design, or YAGNI (you aren’t going to need it).
Another good example is uncle Bob’s bowling score TDD example. I think this is a good example of how a TDD narrative brings you to a clean solution that would have been explicitly unapproachable via an up front design.
To make it a really exciting presentation, up-front you could challenge the audience to design the two scenarios using whatever methods they find appropriate. You then would show the TDD way of designing them.
The real WTF moment for me with TDD was when Beck removed the two subclasses of Money, and the tests worked. This is not a trivial action; the man deleted two classes! The confidence to do something like this can be found only by two means.
1) gathering all the senior players in a code base and running through scenarios, followed by an extensive follow through to confirm it works
2) TDD
=D