From July 31st, 2010 to August 27th, 2010, players of Pokémon HeartGold or SoulSilver can receive the Enigma Stone via Nintendo Wi-Fi Connection. Below is the press release from the official website.

The elusive Legendary Pokémon Latias or Latios will roam around the Kanto region in your Pokémon HeartGold or SoulSilver Version game after you've beaten the Elite Four and claimed the National Pokédex. Now there's a way to catch both of the amazing Pokémon in your game! Receive the Enigma Stone via a special Nintendo Wi-Fi Connection distribution, which will lead you to the chance to catch Latios in your Pokémon HeartGold Version or Latias in your Pokémon SoulSilver Version!

Similar to the other half of the Legendary pair you can catch normally during gameplay, the Pokémon you'll get via the Enigma Stone will be at Level 40. In exchange for the Enigma Stone, you'll also get the Soul Dew, a held item that powers up the Special Attack and Special Defense of Latios and Latias!

Source: http://www.pokemongoldsilver.com/us/#/news/enigma-stone