Update: The Pokémon Company just got in touch to let us know that Pokémon Black/White compatibility was mentioned by mistake. Deoxys will only be available for Pokémon Black/White 2.
Mythical Pokémon Deoxys will be available via Wi-Fi distribution for Pokémon Black/White 2 next month in the U.S. From May 8th through May 31st, you’ll be able to add Deoxys to your games via the Nintendo Wi-Fi Connection.
Additionally, Deoxys is also headed to the Pokémon Trading Card Game as a powerful Deoxys-EX card in the new Pokémon TCG: Black & White—Plasma Freeze expansion, which will be available in stores in the U.S. on the same day.
Once you have Deoxys in Pokémon Black/White 2, you can bring it to Nacrene City to have it switch between its Normal, Attack, Defense and Speed forms. Each one has a distinct appearance. More information on this Deoxys will be made available at a later date.