Very plausible, probably won't effect price at all. I hope to see this implemented into all devices. The iPhone has been following suit on a lot of DS(lite/i) trends, I suspect the next being a front facing camera and wouldn't be surprised if they pulled this trick out of their hat.
It doesn't seem like a too complicated process, the camera and motion sensors are probably all it requires then a 100kb script of code.
The price will most likely be 169.99 I'm guessing, maybe. 149.99 to try not to compete with the Wii, however.