Pokemon Black and White isn't intended for the 3DS. Keep in mind that even though Gold and Silver were labelled "Game Boy Color" they were still games for the b&w Game Boy. And it took over 2 years for a Pokemon game to be created for the DS (Diamond & Pearl), so I wouldn't expect anything better for the DS.
This is true but the dev cycle on the Pokemon has been drastically reduced, Diamond and Silver, Platinum, now HeartGold and Soul Silver Have all been released in short time. I do agree with you and think it will quite possibly not be a 3DS title but at the same time, with the launch lineup or I should say titles that will launch shortly after. They are very aggressively pushing the 3DS. So it is not out of the realm of possibility that the first 3DS pokemon is in fact Black and White
Gold/Silver came out in Japan in 1999, Crystal was released in 2000 (JP)
Ruby/Sapphire came out in Japan in 2002, Emerald was released in 2004 (JP)
FireRed/LeafGreen came out in 2004 (JP and US)
Diamond/Pearl came out in Japan in 2006, Platinum in 2008 (JP)
HeartGold/SoulSilver came out in 2009 (JP)
That's still four years between generations. The other games are remakes/ports, so the dev cycle is going to be shorter for those. But for the main games? They would have to had known about the 3DS 3 years ago.
The best argument, however, is that the 3DS will still be relatively new when Black and White comes out, and it makes more sense to aim for the higher user base (the DS) for better sales.