Personally, if I have a good, mild sore, I'll work out but do a different kind of workout. Right now my chest is nicely sore from yesterday's bench presses, so I'll do some cardio today. I'll be doing the necessary arm movements but probably not in the same range of motion as Cathe. I've heard some folks say that moving your sore areas can help clear up lactic acid. And from personal experience, I know that's true. However, if it's a mean, bad sore I rest. If my body says it can't do any more, I listen. I've learned early on that if I don't listen, my energy is shot, my form suffers, I can't do the moves properly or finish the next workout, and it only makes for more soreness.
Hope this helps.
Pinky