From my own experience it does shorten your Achillis tendon and it can happen quite fast actually. I never used to wear heals but for the last few months I decided to be a little more of a "lady" and wore heels a lot. Now I can feel the stiffness in my heels and in the mornings I walk like a old lady!!!!!

Use heels in moderation and do strectch your Achillis tendon regularly =)