There are actually lenses in the Google Cardboard things. I don't entirely understand why: I think they spread the image out to fill out more of your field of vision.
The video player "pre-distorts" the image. It doesn't just display the video as encoded, it splits it into two images and kind of "rounds" each one out, so it ends up in the middle of the lens on the Cardboard, which spreads it back out again.
It actually looks nice. It was more complicated than I thought it would be.
Things "just work" though after being encoded by BD3DMK3D: the video players detect everything automatically.
Thanks!