Gen 1 (Gameboy and Gameboy Pocket) - Red + Blue + Green (but Green was never released in English), then Yellow
Gen 2 (Gameboy Color) - Gold + Silver, then Crystal
Gen 3 (Gameboy Advanced and Gameboy Advanced SP) - Saph + Ruby, then Emerald, then FireRed + LeafGreen
Gen 4 (Gameboy DS and Gameboy DS Lite) - Diamond + Pearl, then Platinum, then HeartGold + SoulSilver
Yes, everyone has noticed this pattern; it's just the way Pokemon games are released. Release two versions (in the case of Gen 1, three versions) of the original game with certain Pokemon that can only be acquired in one game or the other, then release a third version of the same game a year or more later (with the exception of FireRed and LeafGreen) with upgraded technology in the game and more features, for the point of releasing another game, getting more use out of the previously created Pokemon and storyline, while the developers work on the next game's Pokemon and storyline. It's no different than offering an expansion pack for a game while the developers work on the next big version of the game. :P I doubt there will be a third release to SoulSilver and HeartGold. They'll probably do the same as they did with Gen 3 and flow straight from the two remakes into Gen 5. Though, if the pattern holds, there will be a new Gameboy out by Gen 5 too. :|