During the transition from the Gameboy to the Gameboy Color, there were a string of games that were "hybrid" playable on both the GB & GBC. Normal GB games played on the GBC had only 4 shades of 1 color (and you could change that color via system options), while GBC Only titles displayed a full range of colors but were unplayable in the original Gameboy. The "hybrid" games had less colors than GBC Only games, but were playable on both handhelds. Practically all normal GB games between the market transition from Gameboy to Gameboy Color were hybrid games that could be played on both (so that publishers could sell to both handhelds with a single game).
Why am I saying this?
Because I imagine something similar will happen with the DS and the 3DS. I believe there will be "hybrid" DS games that will play normally on the DS, but when played on the 3DS the handheld will render them in 3D. Those games might not have any extra 3DS extras other than being rendered in 3D, but it still sounds like a great investment. Imagine buying a DS game in the next year, and then when the 3DS comes out, playing that game again with (quite literally) a new perspective.
"A closet intellectual, he acts dumb to impress women." I type random stuff in this line if I have something extra to say.
Yeah there were a lot of hybrid games during th GB, SGB, GBC, GBA, DS, & DSi era.
Game Boy games with Super Game Boy support like Donkey Kong and Space Invaders were prime example of this as was Link's Awakening DX, Dragon Warrior III, Dragon Warrior Monsters 1 & 2, Pokemon Crystal, The Zelda Oracle games, all the Pokemon GBA games, and a ton of others too. It'll be great if we could still transfer data from Golden Sun 1 & 2 to Dark Dawn but only if the original groups are in the game.