There are a lot of excellent posts, so I'll try keep mine brief, but I did want to make a couple of points. My information comes from our pediatrian and we had a similar discussion at my son's 6-month visit.
Vitamin D-My understanding on vitaminD, at least according to our pediatrician, is that there have been some cases of ricketts in climates with less sunlight (I live in Seattle) but only in cases of dark skinned babies. Light skinned babbies are probably fine; however, there are some who think there may be subtle changes in the bone even in light skinned babbies. There is no documentation of this that I am (or my pedriatrician) aware of, but some advise admistering it as a precautionary measure. Our ped left it up to us, we did not supplement.
As for iron - breastmilk does not contain a lot of iron, but what it does contain is extremely well absorbed by the body, so exclusively breastfed babbies do not need any additional iron. However, foods containing nitrates do interfere with the bodies ability to absorb iron so it becomes a concern once you start your infant on solids. In general we avoided store-bought baby food, but made an exception when it came to nitrate-containing foods because commercial baby foods do go through a process to filter out nitrates. We also do try and get some iron-fortified foods (such as oatmeal) into his diet each day, but no supplements.
Fluoride - Our ped reccommeded 2 ounces of tap a day. Flouride can be a problem for formula-fed babbies who can get too much if their formula is mixed with fluoridated tap. To be honest, I followed this advise somewhat inconsistantly. Very small babbies can get cavities, but we asked our dentist who said that it is uncommon if the teeth come in with large gaps inbetween (as our son's did).
Hope that is helpful.
-Camille