This guy cracked his stress fracture, too, but he kept racing:
http://www.cbsnews.com/8301-33747_162-57490227/u.s...
I think it's a challenge.
Mad props to this guy for persevering through his race, but he did break his fibula. I broke an actual bone that carries my weight, not the slenderest of all long bones. Still, that's really impressive, particularly given the discomfort associated with a fracture.
When I cracked my femur during a college track workout, I finished the last 150m of the 400 I was doing. It wasn't a complete break, though I did hear and feel it pop. I shiver to think that I walked around on it until the next day when it hurt even more and I finally had it looked at.