My understanding of the 3DS's auto stereoscopic display is that it uses blockers to hide half the screen from each eye. However, I've noticed that when direct sunlight is on the screen, the effect seems to die, showing both halves of the view at once (as if you're watching from outside the valid zone). What's up with this?
I've been searching online for some documentation to back up my educated assumption, but I can't find anything to verify my thoughts. Alas, here is my two-cents based off of my understanding of the technology.
The screen and overlay are designed in such a way that two images are vertically interlaced together on the LCD and a parallaxing overlay splits the image. The LCD is back-lit, so it shines in (mostly) one direction, and the overlay diverts the light in alternating directions for each eye.
However, when you take your 3DS outside, you now have two light sources-- one that is shining from the back-side of the LCD and the sun. The sun-light bounces off the back of the LCD and is reflected through the parallax overlay. This should distort the image, which is probably what you are seeing.
Again, I can't find enough technical information to verify that this is happening but considering the physics of the display and LCD technology, I'm fairly confident that this is the reason why this happens.