Also, Gold/Silver can be played on ANY Gameboy, even the really old ones, but it had enhanced features when playing on a Gameboy Color. Crystal could only be played on a Gameboy Color.I think it'll be something like for Pokemon Crystal. Crystal came out (at least in the US) around the time when the Gameboy Advance(d?) came out. You were able to play the game on the GBA with a wider, rectangular screen, rather than on the Gameboy Color, which had a small square screen.
I'm guessing that it'll be like that for the 3DS. The 3DS will certainly be able to play BW, so it'll probably just provide for a better visual experience.
And because of that, I speculate that Black/White will have DSi/3DS enhancements, while the 3rd version will probably be DSi/3DS only. I'm not really sure if there's any DSi only games, though... are there?