After having researched the Vitamin D thing, I don't agree with Dr. Newman's advice at: kellymom.com.
Even though someone may not have rickets, they can be deficient in Vitamin D. The current toxicity levels are also way too low...it would take many, many more doses to overdose on Vitamin D. DH just went to the doctor to get his levels checked, and his doctor said the same thing.
Some really great resources are from doctors/scientists at:
http://vitamindfoundation.orgwww.vitamindcouncil.org
The former has a great video presentation by a Vitamin D expert--Dr. Vieth--about the biological/epidemiological basis for upping Vitamin D intake and Vitamin D levels ("Prospects for Vitamin D Nutrition"). Current wisdom states that if your levels are between 30 & 100, you are " normal". However, levels should really be around 80 and above to store Vitamin D, which most of us in the US and Canada need to do because we don't get the necessary UV light during the winter. The presentation explains why.
Low D is associated with increased illness, including cancer.
Anyway, I would start a newborn right away with 400IU.
I supplement my 9mo with 400IU/day, me at 6,000IU/day, and my 4yo at 1,800 IU/day, although this may be low for her. My 4yo and I were tested and she is below 30 and I'm at 32. We're going to re-test in a few months. The baby will get tested at her 1 yr appt.