I've got a friend who got a front tire flat on the descent of Ebbetts Pass in the 2002 Death Ride. 15% grade. She managed to stay in control and not crash - I have no idea how, but I have a tremendous amount of respect for her.

I think the key would be not to panic and to try to reduce speed slowly while riding on the rim. I also think this is an area where clinchers may have an advantage - you still have rubber on the wheel, whereas tubulars may tend to roll off (a la Joseba beloki in the 2003 TdF).