I dropped my 12" Powerbook, which fell four feet and landed square on its corner. All it did was press in the corner of the case and buckle two small places in the bottom shell. All of the hardware survived, passed all the hardware tests, and functioned perfectly.
I've heard similar stories, as well as one lady baking her Powerbook in the oven and one guy running over his with a Toyota SUV. They all survived.
A friend of mine dropped his iBook similar to the situation where I dropped my Powerbook, but his outcome was a bit different -- the plastic on the corner shattered.
That's not to say that the PB is stronger than the iB -- just that your mileage may vary. The real idea is to keep stuff like laptops as far away from the edge of your desk as possible.