The goal is to do 3000 ab exercises in 35 days, which breaks down to 100 reps six days a week. If you sign up for the challenge I believe you'll get videos/links twice a week with new exercise ideas (scroll down to see a video with three exercises). Though you may choose to do crunches or sit ups, the videos will offer alternatives.
I also read that when doing the plank every six seconds counts as a rep, so one minute would be 10 reps, and 10 minutes of plank would meet your 100 rep goal for the day.
I think this is great challenge! Honestly, ab exercises do not take that long and therefore can be quickly knocked out during my day.
I want to be on track so I'm going to do 100 crunches right now as I watch the Casey Anthony trial weekend update (um, confession: this trial has been my obsession).