There's a school of thought that abs are like every other muscle group and need a 48 hour rest between workouts. I had a chance to hear the head trainer for all of Ballys gyms talk about abs, and he said that working them every day causes micro-scarring in the muscle, which will keep them from strengthening and developing.
Really, what's the point of doing abs every day? Three times a week is plenty to strengthen your ab muscles. And Jessica has a good point that you need to work your complementary muscles (lower back) as frequently as you do abs so not to develop a strength imbalance that can lead to injury.
There's a saying among bodybuilders that "abs are made in the kitchen". What that means is that six-packs and rock hard abs come mostly from what we eat (or don't!) In reality, we all have six-packs but they're covered by fat so aren't visible. Great abs come from low body fat -- and low body fat comes from clean eating and cardio to burn calories. So, in my opinion, the best exercise for abs is cardio, not crunches.