NYT has an interesting article on vitamin D and performance.
A few interesting facts are mentioned. One that surprised me is that in a study, long-distance runners from Baton Rouge, Louisiana, had low levels of vitamin D. If they have low levels of vitaming D, then us Seattlelites have not hope :-)
Additionally, the articles says that athletes seem to be fittest in August, when ultraviolet light is strongest. So sign up for a race in August!
I am too worried about skin cancer to spend long hours exposed, but, just like in all things, it seems like a good balance of sunshine and protection is needed.