A new Pokemon title could be slated for announcement soon.
Pokemon Black & White might not be the only new Pokemon games that the world will see in 2011. Japanese magazine Dengeki appears to claim that the announcement of another new Pokemon game is coming in May.
The wording isn't explicitly clear, but Andriasang translates a blurb in the magazine to say that the July issue of Dengeki, which is released on May 21, will include "New Title Information" for the Pokemon series. When this kind of phrasing is used, it tends to refer to a new game.
Andriasang adds that a Pokemon title would likely be announced through a venue other than Dengeki, so if the blurb is referring to a new Pokemon game we could hear about it earlier from another source. Now that the possibility is there, we can at least speculate.
Black & White were released in Japan on September 18, 2010, and in the rest of the world in early March 2011, so the reveal of a main entry is incredibly unlikely. However, it seems like the series is due for another spin-off. This could mean that we'll see another Pokemon Ranger title, a Pokemon Mystery Dungeon title, or even something like Pokemon Pinball or Pokemon Trozei!. More likely than not, a new title would be announced for the Nintendo 3DS.
There have been multitudes of Pokemon spin-offs released over the years, so there's just no predicting what could come next. I'm going to go ahead and say that if something is announced, it'll either be that Pokemon MMO fans have been dreaming of for years, or a Pokemon flight simulator.