It seems like we these "health researchers using Wii in study" stories about once a month but, anyway, Oxford University, last I heard, was a pretty good school. And they're looking at the Wii as a means to help stroke patients relearn simple tasks.
The study is a little more subtle than other research involving the Wii (training surgeons, for example) — the motion sensors might be useful for monitoring patients' progress in physiotherapy. Apparently, one of the biggest challenges patients face is constant discouragement from not being able to see, or really understand, the progress they're making. The motion sensors can also tailor exercises to a patient's range of motion.