To be fair, I don't think another system in history has done 400K month after month like DS has started doing. It may be in shape to have a bigger year this year than any system in history.
Its honestly not much bigger in Europe. GBA, for whatever reason, never took off in Europe compared to original GB, which did similarly in Europe as America. DS is returning to the GB pattern. But its going to be nigh on impossible for a handheld gaming system to take off in America like DS has in Japan. The only people who take public transportation are kids to school. Its quite a feat that DS is selling as well as it is--likely in shape to beat GBA lifetime sales. Its possibly because it has the Pokemon equivalent for girls in Nintendogs.
And I think we're likely to see Wii become much larger than DS in America, and much larger than Wii in Japan. Every college dorm, every sports bar or bowling alley, every break room, everywhere where people might want to relax, congregate or party is going to get a Wii. And I don't think its as common to throw house parties in Japan as America.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
