Ok, 307 is right that sit-ups do put unnesasary strain on the back and is unhealthy to a certain extent. With that said my grandfather did who knows ... 300 thousand sit ups while in the military. He is 86 now and he just built his house last year with the help of my father and I. So doing a few is not going to kill you. There is also a difference between acute and chronic injuries. It's unfair to say that sit ups are the sole reason for the degradation of people's lower backs, there is to many unknown factors in people's everyday life, improper lifting, lack of exercise and the fact that more and more people are sitting at a computer for 8 hours a day without proper lumbar support.
I've seen more people that are injured doing squats or deadlifts improperly then anything else.
The reason lower back pain is a leader in disability is people are lazy, fat and it is extremely difficult to prove people don't suffer from severe back pain.
It's a difficult thing to figure out, what really caused the back injury? The long term degrade or the traumatic instantaneous injury.