Current thinking on vitamin D replacement suggests that very high doses are easily tolerated. The risk of overdose is minimal. I was also taught that it is lumped in with the fat-soluble vitamins. However, it actually functions more like a hormone. Most articles suggest an upper limit of 10,000 iu/day, but even then admit that higher doses have been given to patients without complications. The body uses about 10,000 iu/day in its normal processes, so you either have to get sunshine or replace it somehow.
The numbers on who is at risk and who would benefit from replacement are staggering in the medical literature. I'm seeing 60% of the US population and up to 93% of those over 60 are found to have some degree of hypovitaminosis when serum levels are tested. Of course, not everyone will be symptomatic, but since the symptoms are so non-specific, who knows what improvements might be possible in these at-risk populations.
There are many resources, but start at
www.vitamindcouncil.org
Also, go on Medscape or PubMed to get some of the newer peer-reviewed stuff. It's not conclusive yet, but evidence is mounting.
Hope this helps.
-tc-