I have been sitting on mine for months now, but not heard of it inducing labor. Midwife told me to sit on it and bounce up and down and well as circling and it will help baby to get in the right position. Apart from that I find the ball more comfy than a chair.
I have one that I plan to use for laboring positions during early labor and comfort at home in general, but like PP, I've never heard of it as an induction method.
Re: Yoga ball!!