A press release by Nintendo Europe confirming the European release date for Pokemon Black and White versions has also revealed that Shiny Raikou, Entei and Suicune will be available for download over WiFi from February 7th, 2011. The three beasts are currently being given out at GameStop stores across the US, and this new distribution will allow fans from other countries to receive the Pokemon. The press release also confirmed that a Celebi event will follow.
It is unknown exactly how the Pokemon will be distributed, either one at a time or simultaneously. It has not been confirmed whether North Americans will be able to download the Pokemon from WiFi; however, similar events in the past have been available to the US as well.
When any of the Shiny beasts are transferred to Pokemon Black or White, a Zoroark can be obtained. The Celebi unlocks the Ilex Forest event in HeartGold and SoulSilver and can also be transferred to Pokemon Black or White to obtain a Zorua.
Source: Pokemon World UK