In commemoration of the 10th year anniversary of Pokemon Gold and Silver, the official Japanese Pokemon website has announced that a level 5 Mew will be distributed through Wi-Fi from November 11th to the 23rd to HeartGold and SoulSilver players. It will be inside a Cherish Ball, holding a Premiere Ribbon, and will only know Pound. From January 29th to February 11th, it will also be distributed in the same manner to Diamond, Pearl, and Platinum players.
The website also announced that from November 27th to January 11th, the Mysterious Crystal item can be downloaded through Wi-Fi as well. When you take it to the Pewter City Museum, a scientist will reveal that it is actually the Soul Dew. As you leave, Latios (in HeartGold) or Latias (in SoulSilver) will appear and battle you.