Yeah, I'm not sure I'd walk out -- depends on how bad I wanted it I guess -- but I would point out that I tend to write marginal code first and then go back and make it pretty in a second (or sometimes third) go-round, later. It's like that funny old saying, "I apologize for the length of this letter; I didn't have time to make it shorter."

But, if they're just trying to get an idea of your abilities and critical thinking skills and coding style, it's really not that big of a deal.