Kinetic charging.
Upside - battery life
Nowhere near enough energy creation for the iWatch, AND the hardware necessary to do so is large.
Downside - carpal tunnel surgery
What does does wearing a watch have to do with carpal tunnel syndrome?
Carpal tunnel arises from inflammation of then tendons that attach the wrist flexors to the carpal bones. The inflammation causes impingement of the median nerve, leading to the symptoms of carpal tunnel. Further, the inflammation to the tendons likely arise arises from a genetic predisposition, rather than the tendonitis caused by overuse (keyboards, writing, etc.).
Even more, the kinetic charging (that won't be included in the iWatch) supplies energy as the arm is swinging while walking (imagine a pendulum), not by flexing your wrist against an imaginary dumbbell.