Tracking a image and working out its plane of alignment is possible with a single camera.
So what you see on the 3DS is certainly possible, and has been done, on both Android and iPhones. Theres a few apps that even use Dollar-notes as markers, so people dont have to print anything.
Check this blog for lots of examples of AR stuff, most phone based;
(The Qualcomm demos show a lot)
What the 3DS lets you do...much like the Kinect...is have true occulusions. We dont see this on Nintendos demos yet, but its possible.
You can have virtual objects that go both behind and infront of real world objects. Without a sense of depth all AR stuff is "pasted on" and wont ever go "behind" real objects.
This is really a huge advantage for realism, as well as making more advanced interaction possible.
Also, two other things;
a) I'm part of a group working on a open AR system (arwave.org).
b) Dennou Coil rocks. Seriously. Watch it.
Its a great anime, set in almost totaly realistic very very near future where Augmented Reality technology has become common place.
Its also funny, and has a great story that wraps up neatly after 1 season.
( Edited 27.02.2011 13:27 by Darkflame )