I would start small to build a foundation and work from there. One very safe one is if you have a large stability ball. Sit on it and rotate your pelvis in circles, first one direction and then the other. Start with ten circles each direction and build from there, adding sets over time. Yoga can also help stretch and strengthen core muscles. Do those sorts of exercises for a while before moving on to crunches and more intense ab workouts.

Jason,
My surgeon taught me a safe way to start to build muscle (I am hernia prone so I need to be careful)...laying on your back you take very deep breaths, pulling the air down as low as you can, filling you abdomen and stretching it out as far as you can....hold for 5-10 seconds and release....blow out the air completely while pulling in your abdomen as tightly as possilbe...until it is totally flat (concave?) and tight...hold 5-10 seconds and repeat...It works wonders to build abdominal strength...eventually you can start doing it while sitting or standing (I do it at red lights!) or while laying in bed or watching tv...once you have mastered it you can see the difference within a week or 2...
Sharon

If you can tolerate something more intensive than what has been mentioned - I have one of those exercise machines that utilizes your body weight for resistance (can't remember the name of it). It consists of an inclined bench, so for greater resistance you increase the angle of incline. Anyway, for sit ups I lay on the bench at an angle (my head above my feet!) I find comfortable. This provides a meaningful workout but reduces stress (including on my back) quite a bit. I don't think you need to buy the apparatus, just find something that allows you to do sit-ups at an appropriate angle.

Another tool is to get an ab roller. It is one of those simple (and inexpensive) curved metal things with the headrest on them. The headrest provides good support for your head and neck which makes abdominal exercises much less of a strain. I'm sure there are other gadgets out there, but these two are what I have had personal experience using.