If you're expecting to catch a Pikachu in the latest Pokemon game heading to the DS next year, you'll be out of luck because for the first time ever, only new Pokemon will be used in the new title.
Nintendo has introduced new Pokemon in every sequel to the "colored" Pokemon games, which date back to the original Game Boy. However, in every one, the new Pokemon were thrown in and mixed with some of the classic Pokemon as well. That won't be the case in Black and White.
In a blog entry at game developer GameFreak's website (Japanese), studio representative Junichi Masuda confirmed that people who play through Black and White will not see a single Pokemon that they've seen before.
Developers want to give players an experience similar to what they felt when they played through their very first Pokemon game, where everything was new and exciting.
Of course, at some point in the game players will have the ability to trade Pokemon between their Black/White game and their other Pokemon DS games, so every one of the nearly 500 Pokemon will still be programmed into the code, but more than half of them won't be actively put into the game.
This will most likely bring up a mixed reaction from gamers, especially those who have a particular attachment to some of the old-school monsters, but I personally think it's a great move on the part of GameFreak and it should really take away from the monotony that the series has suffered from for years.
Pokemon Black and White have not yet been assigned any specific release window in the US but should be available some time next year.