I think it's funny that people think that 5 million is a stretch. The system will have a heavy launch window lineup of games thanks to Nintendo releasing dev kits to developers early, not late. For it to only sell 5 million to March 2011 tells me that they may delay the system in the US until 2011. I can't see Nintendo delaying the system past the holidays for Japan or Europe as they're the regions that are seeing the massive software drops.
The other reason it may only reach 5 million is that Nintendo will price the system at around $200 while dropping the price of the DS line across the board. Something like this (prices USD);
DS Lite $99.99
DSi $149.99
DSi XL $169.99
3DS $199.99
3DS sitting at $199.99 while selling the same as the DS did at launch will mean it outperforms it considerably as the original DS launched for $149.99. I think Nintendo will try to emulate the hardware run of the DS with the 3DS and avoid having it sell like the Wii. What I mean is that they'll give it a premium price at launch so that it doesn't peak too early in it's life, they still want the DS line to continue to sell and so they'll ease the 3DS in slowly.
Honestly though, I think it'll sell closer to 6 million than 5 million if it launches in November this year in all territories.
