Abdominal exercise with bad back?

Good idea. Many physicians believe strengthening the abdominal muscles can lessen occurance and/or severity of back pain.
Usually recommended. Strengthening abdominal muscles is a good way to help prevent back pain. If you're having back pain now see your doctor to make sure it's ok to exercise. Usually it is. Here's a link with videos of abdominal exercises for back health: http://www.Acefitness.Org/exerciselibrary/exercises.Aspx?Bodypart=1. If you're not used to exercising start with the easy ones.