I would still question your form. I've never came across anyone who I wasn't able to have perform a plank of some kind pain free, regardless of back condition. However, I see it butchered all the time and basically turned into a lower back exercise. It's very easy to do wrong especially if you have a lot of the common issues going on like anterior pelvic tilt.
I wouldn't advise attempting rollouts etc if you can't do a pain free plank because roll outs are like a more advanced progression.
Planks are the foundation to most exercises. Just about everything you do is a moving plank. I would make it a priority to workout why you experience pain with these. I imagine you will be doing them in a less than optimal position and also have an extremely low tolerance for extension (which is why sit ups will feel fine - for a lot of people it's the opposite, it's flexion that hurts). Also because something "feels" fine, doesn't mean it's actually any good i.e. feeling fine is poor reasoning in itself to do something.
I love the following video with Dan John coaching the Swing because, as well as just being typical Dan John quality, he really explains why the "plank" is so important in practical terms. If someone can't plank pain free, I would personally be very hesitant to load them up in any exercise that involves standing up.
He starts to talk about the plank 1-2 mins into it but it's worth watching the whole thing.http://www.youtube.com/watch?v=tVEReOq5Jgs
Also, if it helps any, I typically teach the plank like this,
Lay down on your front (prone). Bring your elbows under your shoulders (your hips legs and abs will still be on the ground).
Lock your legs and get ready to get up on your toes but, don't yet.
Tense your abs and glutes like your life depends on it.
THEN lift your hips up.
Also, face the ground, not forward. If you extend your neck your lower back will try and follow.
Lastly - do the planks hurt instantly or only after a certain amount of time?