What is the best way for people living in non-sunny, northern climates to metabolize vitamin D in the wintertime?

I’ve been reading much on the virtues of vitamin D as they apply to depression, arthritis, osteopenia, energy, mood, diabetes and even cancer; one prominent researcher believes that 20–30 minutes of unprotected sun exposure 3 times per week is what’s necessary to maintain adequate vitamin D levels. However, if you don’t live in a sunny location, what is the best way to achieve this? I know there is a vitamin D lamp that emits UV rays (www.vitaminduv.com) and also various “blue light” therapy devices (which “activate” retinal photoreceptors), but beyond this, is moving to a tropical or sunnier latitude the only solution?