Pokemon who get their own entry in Wikipedia are pretty rare, there being 646 Pokemon available at this point, so Mewtwo is pretty important in the Pokemon mythos. Nintendo says about the impending Mewtwo event:
The legend of the Genetic Pokémon Mewtwo has grown ever since it debuted in the Pokémon Red Version and Pokémon Blue Version games. The Psychic-type Pokémon has always been considered one of the most powerful Pokémon available, and also very tough to catch.
You’ll have a new opportunity to get the Legendary Mewtwo once again! Starting on February 12th, you can receive Mewtwo for your Pokémon Black Version or Pokémon White Version game. Take part in a limited-time distribution event to get this amazing Pokémon. This Mewtwo is incredibly strong and fast, and it even has an exclusive move that Mewtwo cannot normally learn!
However, while Nintendo is being coy, Siliconera is reporting that they have already done this same download event in Japan and they are unlikely to change things up for the distribution in North America.
This is not an in-store download. You must connect to the Internet via your Nintendo DS using the WiFi option in order to download Mew. Instructions on how to configure your DS are in the instruction booklet that came with Pokemon HeartGold and SoulSilver. (page 35)
Well, you encounter Shaymin. Actually catching Shaymin is up to you.
Shaymin had previously only been available via a Toys R Us download event for Pokemon Diamond and Pearl back in February, before Pokemon Platinum was released. You could then trade Shaymin to your copy of Pokemon Platinum after it became available.
Now Nintendo has opened up a method to obtain Shaymin directly in Pokemon Platinum.
However, you have to have a Nintendo Wi-Fi account setup to be able to get Shaymin. And, of course, there is a time limit on this option, though it is a pretty generous time limit.