My bet is if it is on the 3DS, it'll be on the DS, but when played on the 3DS it gets 3D features and extras or something. I mean seriously, it'd be a mistake to not release it close to the 3DS, it'd print money so easily.
This particular game has already been confirmed to come out on the regular DS some time before the 3DS gets released - remember, the confirmation was for fall 2010! Still, I wouldn't discount the possibility of bonus 3DS features.
Creator of Jay's Journey (see quote!) "Game Series 1 is the best because it set the stage for better games!" - Nostalgia whores everywhere