I'm in physical therapy right now for ankle & calf injuries from running. My doc said stationary bike or swimming, but my physical therapist says that if I'm hoping to get back to running soon, the elliptical is a better way to go. I've found that while sitting on the bike, I'm not using my running muscles. The elliptical does. Of course, depending on the severity of the injury, this might not be good.

The most important thing is to heal. You may have to come back to training at a lower level than you left it, but you don't want to reinjure yourself!