The 3DS screen is closer to one of those lenticular "3d" things you'd see on a ruler or a bookmark as a kid. It uses two interlaced images that are fed to each eye separately. The iPad 2 screen is just a normal screen.
Basically, the iPad 2 demo only gives the illusion of being 3D when you move it, but since your head and hands are pretty much constantly moving it should look "mostly 3D" if it has enough sensitivity.
I'm pretty sure that the 3DS isn't doing any head tracking. If you move your head much at all the 3D effect breaks; you have to be looking practically dead at it.
Yep. The way the 3D effect works, there are essentially physical barriers inside the screen separating the different views being rendered for each eye. Move your head so that you're not viewing the screen straight-on and the barriers don't work properly, ruining the effect.
