Depends on the choice of games.. Making having Nintendo porting, or making new "Touch Generations" games for the iphone makes sense, but i dont think we'd see much in the way of retro, mario, or kid-friendly stuff..
It's all about Brain Age etc. There's a big lot of the "mature" audience who aren't likely to buy a DS specifically for those games, but would buy it if on their iPhone, as it looks like a pro device, and nobody can tell you're "playing games"/trying to stop yourself going senile. 🙂
Then again Nintendo will already have planned the successor to the DS, and it probably has similar tech to the iphone, so whether this would happen at all I'd imagine is entirely dependant on iPhone's market penetration.. If millions do end up with them, that's some nice extra money for them.
BTW I'm just pondering, i seriously doubt this will happen at all, but it doesn't seem impossible.