In the interest of full disclosure, I’m on the fence about whether to take vitamin supplements. There are two big reasons for this. One, when you ingest a nutrient that is isolated in a pill, you aren’t getting all the other beneficial nutrients and compounds that you would find in healthy food. Two, scientific research is showing that with some nutrients, more is not better. Classic examples are some of the anti-oxidant vitamins (beta-carotene, vitamin C, vitamin E), which, when taken in excess, may actually become pro-oxidant in the body (and you don’t want oxidation to be happening in your body).
It is difficult if not impossible to get too much of a nutrient from food (one exception is polar bear liver and vitamin A, but I doubt too many people are eating polar bear liver). It is much easier to get too much of a nutrient from a pill.
One exception to my fence-sitting is vitamin D. As mentioned in my On Nutrition column in yesterday’s Seattle Times, few foods are decent sources of vitamin D (the best sources are actually enriched with added vitamin D). That’s because our skin manufactures vitamin D from sunlight, so that’s our natural source. But in this era of SPF 100 sunscreen, exposing our unprotected skin to the sun is controversial, and for some people potentially unhealthy. For added irony, for optimal vitamin D production, you need to expose your skin to the sun when the rays are strongest, sometime between 10 a.m. and 3 p.m. That’s the same daily timespan in which we are told to avoid the sun in order to protect ourselves from sun damage!
Ah…what to do?
The bottom line is this: Get enough D, but don’t assume is more is better. The Institute of Medicine raised the recommended daily intake (RDI) a few years ago from 400 IU per day to 600 IU per day. The tolerable upper limit (the highest dose that should not cause adverse health effects) is 4000 IU per day. That leaves a lot of room in between. Some experts think the RDI should be raised further, while others disagree.
One reason for the lack of consensus is the lack of “gold standard” scientific studies; i.e., randomized control trials. Most of the current knowledge comes from the biochemical understanding of how vitamin D operates in the body, and from epidemiological studies that show associations between higher levels of vitamin D and lower levels of certain diseases (these studies often compare populations who live in sunny climates with those who do not). 
It will be years before randomized control trials like Harvard’s VITAL study (in which some participants are randomly assigned to the experimental group, which takes supplements, while others are assigned to the control group, which takes placebos) produce evidence to support or refute many of the health claims being made about vitamin D.
To learn more, I highly recommend the “Vitamin D and Health” page on the Harvard School of Public Health website. It has lots of great information, with links to even more information. Another great resource that’s slightly more technical is the vitamin D fact sheet produced by the National Institutes of Health. It goes into a good amount of detail on the pros and cons of getting vitamin D from the sun.