The "How did Nintendo pull off 3D without glasses?" Thread.

[Mr. Fister]

I've heard it uses autosteroscopic imaging to get a 3D effect.

http://loot-ninja.com/2010/06/22/how-the-nintendo-3ds-screen-works/

This seems the most plausible method, as those who had hands-on time with the 3DS said it had a margin of error on it.