Were you told to bring your own system to attack from or will that be provided for you?
Certainly, in addition to the technical skills, they will want to see how well you can report on it. I would recommend reading this blog post:http://pen-testing.sans.org/blog/2012/02/09/maximizing-value-in-pen-testing
I would want to see that you can summarize the findings in a non-technical summary and that you can present the risk appropriately. I would want to see what your thought process is on how you rate risks - in other words, for this purpose, I wouldn't care what you rated findings so long as you provided thoughtful support. I would also be looking at how you tell me to fix the problem.