Nice catch. While interesting, I doubt it's any indication of a redesign.
I'm looking at the ad now, and the size of the screens is just ridiculous. It would add at least a half inch to both the vertical and horizontal dimensions of the screen. (Unless, of course, the screen size is the same but the body of the DS is much smaller.) I don't see how the resolution would work on current DS games unless Nintendo utilized some sort of "upscaling", which doesn't seem like it would work too well.
Also, the "screens" aren't actually screen shots of the game. They're cartoon drawings, which indicates ... well, I'm not sure what it indicates. But I would be more inclined to believe in a redesign if they were screens. I'm pretty sure it's exactly what you said--a mock up to make the screen shots better viewable.
Finally, welcome to the boards! I fully expected an advertisement for cheap video games and iPods when I clicked on the topic.