From <a href="http://nintendo3dsblog.com/there-will-be-no-pokemon-gray">nintendo3dsblog</a>
It's a bit hard to believe, but think of all the adverse effects of not having a 3rd version of B/W. Kyurem will never hog the spotlight outside of Reshiram and Zekrom, there won't be any Move Tutors, new Battle Frontier modes (I loved those in Platinum...), or any chances to re-battle Gym Leaders or most of the in-game Trainers, which is a crushing oversight. Not to mention if the 5th generation ends at Black/White, that means Game Freak will have to put out a 6th gen that much sooner, not counting the time it takes to churn out 27246184364 money-making Gen 5 Pokemon spinoff games. We won't have as many memorable years as we did with the Diamond/Pearl generation. When the D/P anime finally ended after, what, four years of Sinnoh?...I actually felt sad, despite the fact I didn't watch most of the episodes after the first three and before the last ten. I did watch all the movies though.
Though, the article and its sources are still open to interpretation. Perhaps there will be a sequel to B/W, it wouldn't make any sense not to especially with 3DS coming around the corner.