I think it's only a matter of time before they do that, since Nintendo hasn't really pursued the retro strategy with the DS yet. But I don't know if they would let you buy the game once and play it on either system.
As for the systems, the DS should be able to emulate the SNES and Genesis, and therefore probably the PCE as well. (Even the GBA could emulate the NES pretty well.) The Neo Geo is a little iffier, more because of huge sizes (DS cards can only be 128MB, though obviously the homebrew scene has found ways around that) than processing power.
If they don't do that, I would think it's for economic reasons rather than technical ones. Homebrewers have already made programs that download and run other programs, so there's no reason one of those programs couldn't be an emulator. Except that Nintendo would rather we bought our games twice, of course.