@ Lyn: I think you’re thinking of Helium-3. Deuterium exists on the Moon (and on Mars) but also on Earth. It may be more plentiful on the Moon, but not to the point where it would be worth importing it. Helium-3, though, is deposited on the lunar surface by the solar wind (but not on the Earth or Mars, because of their atmospheres) and it is fantastic stuff when used for theoretical fusion reactors (De-He3 reactions are much “cleaner” than De-T reactions. Tritium is pretty nasty stuff, actually) but those fusion reactors do not exist and there will be no market for lunar He3 until they do. Someday, we will mine the Moon for He3, but that day is a long way off.
Mhh, I read it in some scientific review several years ago, they were saying He-3 deposits on the Moon are too insignificant to be really interesting, as much as De is on Earth (at the opposite of the Moon). Though I noticed that opinions and theories like this sometimes heavily change with time and science on very short periods, so this might be outdated.
Or maybe I remember very poorly, which is also possible.