Hi wildblue_yonder -
I have a herniated disk in my lower back and a little piece of a disk that is broken and resting on some nerves that go into my leg (lovely).
About 6 months ago I was allowed to work out again. My Dr and physical therapist approved me using a recumbant exercise bike and then after I worked up to it a little..the elliptical trainer. If you havent heard of it, its a low impact machine that burns a ton of calories. kind of like a stair master and a treadmill all in one.
Of course, Im not a doctor so I dont want to give you bad advice...but for me, these two machines have been very safe for me to use. And my back has been much better since I've been working out consistantly. In fact, it now tends to act up if I dont work out...I think it starts to get stiff.
Anyhow - just start slowly so you dont reinjure yourself. listen to your back.
I hope this was helpful.
good luck!