A press release from TPCi has announced that the Enigma Stone will be available through Wi-Fi download starting this Saturday, as you can read below. The item allows you to encounter Latios in HeartGold and Latias in SoulSilver, and then itself changes into the Soul Dew.
Beginning this Saturday, July 31, through Friday, August 27, players of Pokemon HeartGold and Pokemon SoulSilver Versions who have wireless broadband Internet access can receive the Enigma Stone for a chance to catch the amazing, yet elusive, Legendary Pokemon Latias or Latios.
Similar to the other half of the Legendary pair that can be caught normally during gameplay, the Pokemon encountered with the Enigma Stone will be at Level 40. In exchange for the Enigma Stone, Pokemon fans will also get the Soul Dew, a held item that powers up the Special Attack and Special Defense of Latios and Latias. Only Pokemon players who have beaten the Elite Four, claimed the National Pokedex, and have two or fewer Wonder Cards can solve the riddle of the Enigma Stone.