As I have mentioned. The book The Harvard Medical School Guide to Tai Chi by Peter M. Wayne PhD. is an excellent book and well worth the read. (Use our Amazon Search to buy yourself a copy)
Here is an excerpt from page 119.
" The rich diversity of Tai Chi's movements — the sequencing, timing, and combinations of different muscle groups— provides excellent training for the coordination of neuromuscular patterns. Research supports that Tai Chi can improve your dynamic balance as you move and help you recover from perturbations in balance, for example, when you slip on a wet sidewalk. "
Later on page 121 the author speaks to Tai Chi and Parkinson's Disease.
"A study published in the New England Journal of Medicine shows that Tai Chi can improve both balance and movement control for people who have Parkinson's disease. "
Pretty cool IMHO