Gerudo Ganon said:
Considering they've had the Red and Blue versions of Mystery Dungeon on two different consoles (GBA and DS respectively, if my memory serves) it isn't completely out of the question to assume that half of Gen V may spill onto the 3DS - I'd consider it more than likely myself that Black and White will be on DS, and every other Gen V game appears on 3DS.
This is almost certainly how it will pan out. The problem with B/W is that we're at the stage where a new generation needs to be out there. If we had to wait any longer, the 4th Generation would be, by far, the longest yet (as it is, they were a couple of months short of that accolade anyway).
The fact is, though, that the hardware wasn't there for Gamefreak to be able to place B/W onto the 3DS. B/W will quite easily have been in development since D/P was wrapped up (head producer, Junichi Masuda stepped down from Platinum and HG/SS development, more than likely, to begin work on B/W's long development process), and by the time the actual software development will have started (approximately a year and a half ago would be expected), the 3DS was still very much a concept. We know that the earliest any developer got their hands on development kits for it was well into last year, when Sakurai's Project Sora team got the chance to begin on the new Kid Icarus.
The 3DS and B/W just missed eachother timing-wise. Although, I suspect that because they had to work with the DS again, the experience of the past 5 years has been put to full use and could result in the best new generation launch games ever. We already can see just how much they've put into this and how they're working extremely hard on the polish of the game because the base of it was already there. Something you couldn't really do at the same time as learning a whole new development architecture.
The rest of Gen V will almost certainly be 3DS exclusive, though. I'd suspect the third game to B/W will be similar to Crystal in making the upgrade leap, though I really hope we see the superior power of the 3DS being really put to work. It could certainly make the third game an even more exciting prospect than before if it can genuinely offer us something new - even if it's just a huge graphical overhaul.