Starting today, October 15, 2010, there will be an event for Pokemon #151, Mew! This is to commemorate the 10th anniversary of Pokemon Gold and Silver's release.
The Mew doesn't unlock anything, and only has the move Pound but it gives you the very rare Pokemon!
The OT is FAL2010, and the ID is 10160, so unless that is your ID Number, you cannot rename it sadly.
The Nature is random, so you can soft reset for any nature.
Remember, this event is only for HeartGold and SoulSilver, so don't waste your time trying to get one on your Diamond, Pearl, or Platinum.
The event will be over Wi-Fi. This event ends on October 30th, so get it while you can!