only time I see broken timing belts is from either something siezing, something puking oil all over the timing belt, or because completely ignored the recommended intervals.
Another side note, just like any rubber-component, TIME is a factor. A 20 year old belt that has 2000 miles on it, will be deteriated. Mileage isn't the only factor, although for most car owners, thats not a problem.
The one that you can't forsee easily is pullies/tensioners/cams siezing. For example ford 1.9 and 2.0 SOHC engines are pretty famous for waterpumps siezing. They are non-interference motors though, so its not a major issue. But all the other situations, are very easy to anyone that pays even passing attention to their car.
But then, most people don't. My job security would be greatly endangered if people weren't so lazy/uneducated about their cars, so I'm not going to complain too loudly, although it will always mystify me.