Starting today all people can get the Enigma Stone in HeartGold and SoulSilver over Mystery Gift. Further information can be found on in the original thread:

"The Pokémon Wi-fi event for HeartGold and SoulSilver has finally come to America, Europe, and Australiasia. In this event you will have the ability to receive a Latios or a Latias (depending on the game) by going on wi-fi and receiving the Enigma Stone. Head over to the Museum in Pewter City to receive the Soul Dew and go outside where either Latias or Latios will appear. The wi-fi event starts on July 31, 2010 and ends on August 27, 2010. Make sure to get yours!"