More than likely most of the growth is coming from the DS considering more than one person in the house will own one. However I think we'll see some more expansion in the console world of America due to the Wii as it's probable that not all gamers of a younger are live with their parents or family... or that families before even had a console. I mean it's something that's hard to comment on.
I feel the best way of knowing of someone is expanding a market is by looking at the competition. Why? Well if someone is selling at unprecedented rates then if nothing was expanding then the other consoles would be doing exceptionally bad. However both 360 and PS3 are able to thrive in a market considerably dominated by Wii suggesting that Wii is expanding rather than taking sales away from PS360. Now it's not saying Wii isn't taking sales from them, because they are, but that's not the only place where its generating sales.
