A case study conducted by the University of Medicine and Dentistry of New Jersey and published in the October 2008 issue of the American Physical Therapy Association's journal found that when a teenage boy with cerebral palsy played Wii Sports as part of his regular therapy, "there were positive outcomes at the impairment and functional levels," according to the abstract.
[T]he patient was a 13-year-old male with spastic diplegic cerebral palsy. In a school-based setting, he participated in 11 training sessions, over a four-week period, using the Wii while continuing to receive physical and occupational therapy. The sessions were each between 60 and 90 minutes long and used the Wii sports games software, which offers boxing, tennis, bowling, and golf. He trained in both standing and sitting positions.
“ 'Improvements in visual-perceptual processing, postural control, and functional mobility were measured after training,'” the researchers reported.
Lead researcher Judith E. Deutsch wondered if the $250 console "could provide an alternative to the high-cost, high tech virtual reality rehabilitation robotic systems."
The Telegraph reports on a similar study in Newcastle, where kids with hemiplegic cerebral palsy (where one side of the body is affected) played specially-designed Wii games to encourage them to use their affected arm. According to lead researcher Professor Janet Eyre, "There has been a big improvement in arm function and in hand-eye co-ordination. We're getting them to the stage where, without thinking about it, they use that hand."
People with hemiplegia can have especial difficulty using the fingers of their affected hand; one pilot study using a PlayStation 3 and a sensor-glove (the 5DT Ultra Glove) found that two of the three teenage boys who participated were able to do things with their affected hands that they couldn't do prior to playing the specially-designed games–e.g. using their affected hands to brush their teeth, carry grocery bags, dress themselves, or hold a spoon. The full-text of the report, "PlayStation 3-based Tele-rehabilitation for Children with Hemiplegia", is available as a PDF here. (Note: I had to explicitly right-click and choose "Save Link As" to download it properly).