Two events were announced on the official Japanese Pokemon website today. The first event, beginning November 11th, 2009 and ending on November 23rd, 2009, a Mew will be available to download via Mystery Gift over Nintendo Wi-Fi Connection. Players will need a Japanese copy of HeartGold or SoulSilver in order to receive the Pokemon. Mew is Level 5 and has a Premier Ribbon. The only move it knows is Pound (はたく). Beginning January 29th, 2010 and ending on February 11th, 2010, players of the Japanese version of Pokemon Diamond, Pearl & Platinum will also be able to download Mew.

The second event, Latios and Latias will be available for capture in HeartGold and SoulSilver as players will have the opportunity to receive the Enigma Crystal (なぞのすいしょう) also via Nintendo Wi-Fi Connection. It will be available from November 27th, 2009 to January 11th, 2010 for the Japanese versions of the game only.

Source: http://www.pokemon.co.jp/special/hgss/news/15.html