After the launch of a set of Pokemon games there is usually a tertiary release that expands on the features, Pokemon, and dungeon areas of its two predecessors. However, it didn’t happen with HeartGold and SoulSilver, and according to developer Game Freak, it won’t be happening with the upcoming Pokemon Black and White.
In a recent interview, the Pokemon RPG developer confirmed that Black and White would be the sole releases for this Pokemon entry, and that a Gray Version would not be made for either the DS or 3DS. So if you refused to preorder Black or White for fear that a superior version would hit store shelves at a later date, you won’t have to be worrying about that any longer. Go ahead and place your deposit for either of the upcoming releases.
[Source]