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So where does that leave the "Tamagotchi" phase in the U.S.? 
Not to marginalize it by any stretch, but in comparison, the Japanese market eats up the genre in ways the US doesn't. That's purely by comparison, though, and maybe I should check the sales of the game in Japan before I say things like that, but I still don't think 'Dogs will be the truly "killer app" that AC or MK would be in the US and Europe.
I don't expect 90% of DS users to be online. I think that's a heinously unrealistic number, but I credit Nintendo for shooting for it and I believe they've done so because of how much they intend to streamline the process of getting online. I've read a list of the AC features and one of them is to simply enter the town and name of your friend and you can access their town if they've given you specific permission. In other words, no screwing with IP addresses, no setup beyond talking to the Copper at the main gate of the town and saving your friends to your buddy list so they can enter the town again later.
Again, 90% is unrealistic, but they're going to make it as easy as possible.
I think of MK and AC as killer apps because they'll be the first games which will allow Nintendo gamers to either compete or congregate over the internet in a game setting. The online element of the games could easily determine their success. Players who have been hesitant to pick up a DS would probably be brought in by one of the two, be it the die-hard competition of MK or the community-based nature of AC which just became literally hundreds of times larger and more detailed.
In fact, I dare say it's the brand recognition which will sell these games so well. AC was a hit on the cube and Mario Kart is Mario Kart and has been popular since its inception.
I'm not going to shed a single tear if 'Dogs sends DSs flying off the shelf in the US (with all the dog lovers out there, it could be a coiled spring waiting to be unleashed (again, no pun intended)), but I think the game which I'll be able to place in front of any modern gamer which will sell them a DS would either be MK or AC.
Again, I'd love to be wrong, but that's my current prediction.

-SB