I see a long debate on this one..
D3 can actually be toxic to animals that are not depedent upon sunlight (uv) to process calcium.
Exerpt from
http://www.thegeckospot.com/leocare2_2.html
"Although there is a lot of variation between the results reported from different sources, it is clear that it is not possible to provide an acceptable calcium : phosphorous ratio without supplementation of the food. Dr Klingenberg recommends that leopard geckos should not consume calcium with vitamin D, he recommends using a calcium-only powder 2 times per week and vitamin supplements every other week."
I am of the belief, that nocturnal animals have adapted to calcium absorption without the D3 that UV lighting synthesizes for them.
D3, after being produced, can create an over abundance of calcium, build up in internal organs and fatty tissues, eventually poisoning them.
D3, is often an ingredient in calcium powders, used for day walkers, that do not get a lot of natural light or UV lighting to process D3 naturally.