I thought I'd put my $2 in.

This past year, I did extensive research into vit D, trying to figure
out if it was linked to seasonal affective disorder. Also, this past
Spring, I worked long hours in a greenhouse and DID get a tan! I was
surprised, since I also heard that UV doesn't get through glass.
Some of the best info I found was on the Weston A Price Org webpage.
It notes that 5% of tanning UV rays come through glass- But the
tanning rays are only available between 10 am and 2 pm, and Northern
latitudes do not get enough of the rays during the winter.
More info on a separate email and at the website(s).
http://www.westonaprice.org/basicnutrition/vitamindmiracle.html

I also found interesting info at
http://www.mult-sclerosis.org/news/Jan2002/FullTextVitaminDReassessment.html
stating the 400 IU recommendation was from a teaspoon of cod liver
oil, not what is physiologically necessary. 'Our understanding of the
chemistry of vitamin D now makes it clear that it is not really a
vitamin; the reason we need dietary intakes is only because we are so
deprived of ultraviolet light, which triggers its synthesis in the
skin. A whole-body sunlight exposure barely sufficient to trigger
tanning (the minimum melanogenesis dose, which may be as little as 15
min for pale-skinned individuals) has been established to be
equivalent to the consumption of as much as 10,000 IU of vitamin D.'