Pokémon Diamond and Pokémon Jade are not actual Pokémon games. They are pirated copies of Telefang Power and Telefang Speed that were poorly translated into English by a Chinese bootlegger. Because of the poor hacks, many Pokémon fans who have played Diamond or Jade have been left with negative impressions of Keitai Denjuu Telefang, which are entirely undeserved.
Smilesoft was in no way associated with these translated games and, despite some claims, had no responsibility in creating them after sales of Keitai Denjuu Telefang were supposedly poor. Also, they were definitely not created by Nintendo to gauge reactions to new features implemented in later Pokémon games, such as the night and day system and the ability to run. It is also important to note that the upcoming official Pokémon Diamond and Pokémon Pearl for DS have absolutely nothing to do with the Pokémon Diamond and Pokémon Jade featured here.
Diamond and Jade have a few differences from Power and Speed, notably glitches that were not apparent in the original games. Besides the translation and changed title screens, some changes include the inability to nickname monsters, a glitch that makes adding secret monsters to the directory impossible without cheat codes, and the loss of the ability to pick up vases after the power has been shut off.
Interestingly, the creatures on the box covers and title screens of the pirated games never appear anywhere else. The blue snake monster associated with Diamond seems to have been made especially for this purpose, while the creature featured in Jade's art is actually the forest guardian Shishigami from the movie Princess Mononoke. Also of interest is the fact that a red diamond shown on a 2 in 1 game cart of Pokémon Diamond and Pokémon Crystal is also shown on another pirated game box, Kirby - Red Diamond, indicating that the two carts may have had a similar origin.