An audience-participation exercise in which the audience must completely build a product using XP principles, like pair programming, story writing and estimation. For the exercise "build" here means that the developers will draw a picture of the desired product incrementally. The audience is broken down into groups, the members of each group take on different roles (such as "developers" or "stakeholders") and there are certain rules in place to mimic what it's like to work on an XP project.
There are good overviews of an Extreme Hour here and here.
It's something that's used by XP consultants to explain the methodology. Hopefully it'll be interesting and quite fun.
See you there.