There is no such thing...
It has not yet been announced when Keldeo will be available for download in Pokemon Black Version and Pokemon White Version.
There will be 649.
Tornadus is a version exclusive Pokemon, exclusive to Pokemon Black.
march 6th 2011 in Canada
There is not going to be a Pokemon Grey version. Instead, Gamefreak has announced that they will be releasing two new Pokemon Games named: Pokemon Black Version 2, and Pokemon White Version 2. They are set to come out in Japan in the summer of 2012, and in North America in Fall 2012. Both games are currently available to preorder at Gamestop, however, there is no release date yet.
Fall 2012
Thundurus is exclusive to White version while Tornadus is exclusive to Black.
There is no English Version yet. It will come out late next year.
Pokemon black and white 2 don't have a confirmed date yet but it has been confirmed that it will come out sometime in spring.
Fall 2012
Pokemon white version is a soon to come new Pokemon game that will feature the 5th generation of the Pokemon series. It is said to be realeased in Japan in fall of 2010. Also a black version game will be released along with it.
HeatoGold and SoulSilver come out on March 14, 2010.