A new article in the American Journal of Clical Nutrition (2006:84:18-28) reviews the evidence on how much vitamin D we need. The authors recommend that the USRDA be changed.
Taking 1,000 units a day only gets adequate vitamin D levels in something over 50 percent of the population. So probably, if you want to be safe, you need to take more. As I have indicated elsewhere, there is absolutely no evidence that 2,000 units a day will hurt you, even with the unscientific and paranoid methods used to calculate the current USRDA of 400 to 600 units.