There are rumors that Black and White will have 3DS specific features. Tag mode, video chat, and a few other features in Black and White might be only available with a 3DS. Pokemon Black and White is set to come out on September 18, 2010 in Japan. What if Nintendo decides to release the 3DS in Japan around that time in order to accommodate for Black and White's new features? There was that rumor that the 3DS would be out in one region before Christmas, so maybe it's Japan? And when Black and White come out in the US and Europe, that's about the the time the 3DS would come out there. This is really just a guess, and it's most likely not true, so what do you guys think?

From what I understand the 3DS is going to be a worldwide release around Q4 2010 - Q1 2011, in other words around the holiday season. Any rumor is meaningless because Nintendo never announced any dates for the 3DS yet. As for Black and White, it's going to be a spring release for North America and Europe, so I assume the 3DS might have already been released by then.

I personally think the only reason Black & White are coming out on the DS is because Game Freak probably didn't have complete faith in the ability of the 3DS to sell well enough when they started making them, so they made it for the DS so that if the 3DS flops they can say that the 3DS-exclusive features were more bonus content. Or something like that. Or perhaps the 3DS-exclusive features will be used as an incentive for people to buy the 3DS? It's very intriguing, at least to me ...

that features are available for the 3DS and the DSi

The video chat is confirmed to be available on the DSi. I don't know if DSi can do any sort of tag mode though. We'll just have to wait and see. Wouldn't be surprised if graphics were a bit enhanced on the 3ds as well, even if they didn't go full 3D.

I'm planning on just waiting for Pokemon Gray, that will probably be 3DS exclusive like Crystal was for GBC.