Dear Jan: 
I just stopped in to visit Lactnet this evening and noticed your post. I carbon copied Pörrö at the email address I found on her website. But if that email address is incorrect, please forward this information to her. Thank you!
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Here is a 2003 Pediatrics article (full text) with information on treatment of vitamin D toxicity in an infant: http://pediatrics.aappublications.org/cgi/reprint/111/4/899
Here are two excerpts from my article, "Sunlight Deficiency: A Review of the Literature," that relate to your questions. Vitamin D toxicity cannot occur with sun exposure due to feedback mechanisms that control production. Infants in Finland in the winter will not have made vitamin D endogenously (via sunlight exposure) because of inadequate UVB radiation in winter sunlight at that latitude. Therefore, their vitamin D intake would only be through their diet. Depending on maternal vitamin D status during pregnancy and on infant exposure to sunlight in the summer and early fall, stored vitamin D may also contribute to circulating vitamin D levels.
"The skin has a large capacity to produce vitamin D. Exposure of the entire adult body to the smallest amount of UVB radiation that produces transient, just perceptible skin reddening is comparable to taking an oral dose of 10,000 to 25,000 IU of vitamin D.8, 9 Therefore, sufficient levels of vitamin D can be developed from partial exposure of the body to sunlight well before sunburn occurs. 
Levels of vitamin D vary seasonally among people exposed to sunlight at higher latitudes, where UVB radiation is higher in the summer and lower in the winter.10, 11 With inadequate summer exposure, vitamin D deficiency and insufficiency can result, particularly during the winter.12, 13 However, with adequate exposure to sunlight in the summer, vitamin D can be stored in the body for winter use.14 The lower vitamin D stores of the spring can be replenished with exposure to the higher UVB radiation of summer sunlight." 
"Prophylactic vitamin D supplementation is demonstrably useful for infants at risk of vitamin D deficiency. No known risks of supplementation exist with 200 to 400 IU/day. Supplementation and fortification with vitamin D has been used for decades in many countries. According to the Canadian Department of National Health and Welfare, "It seems probable that the widely accepted figure of 10 µg (400 IU) per day considerably exceeds the true requirements of the great majority of infants, but that amount can be recommended as an effective and safe prophylactic level of intake from all sources."97 Vitamin D intoxication can occur with excessive intake of dietary vitamin D (i.e., more than 40,000 IU/day for many months in normal adults), but not with endogenous production via sun exposure.98" 
Reference: Good Mojab, C. Sunlight Deficiency: A Review of the Literature. Mothering. March-April 2003; 117:52-55; 57-63. Url: http://www.mothering.com/articles/new_baby/breastfeeding/sunlight-deficiency.html
Additional information on vitamin D can be accessed from the sunlight deficiency page of my website, Ammawell: http://home.comcast.net/~ammawell/vitaminD.html
I hope this information is of some use.
Still nomail from Lactnet,
Cynthia
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Cynthia Good Mojab, MS clinical psychology, IBCLC, RLC, CATSM 
Ammawell 
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