The Enigma Crystal Wi-Fi giveaway started about 24 hours ago for all Japanese copies of Pokémon HeartGold and SoulSilver. To get your Latias (as I did) or Latios:

1. Connect to Mystery Gift Wi-Fi and download it.
2. Go to any Poké Mart and pick up the Enigma Crystal from the man in green.
3. Visit Pewter City's museum.
4. Talk to Steven. You'll receive the Soul Dew.
5. Step outside and save.
6. Lati@s awaits.