I'm not a doctor, but my doctor told me when I asked that excess Vitamin D passes through your body harmlessly, so there's basically no reason not to supplement if you're worried you aren't getting enough.
According to the comment thread involving devaboone Vitamin D toxicity isn't inherently due to the Vitamin D, but the impact that it has on blood calcium levels.
I don't think this is true and if you a tendency to form calcifications an excess of vitamin D might be a bad idea.