Initially, I was going to react negatively to this (as a father of two kids under the age of 11), but the more I think I about it, the more I like it. I assume that if I was scheduled for a technical interview, I'd likely spend at least 10 hours preparing for it (with perhaps only a bit of assurance that I had prepared properly). If I were given a ~10 hour contract project, I would know the problem up front, and be free to complete it without the stress of a whiteboard session.