That's fun. Obviously that's not how it works, 1px is 1 physical pixel everywhere I've ever seen. Think of how much would break if something decided to do this "right" ...

On retina displays it is two pixels.

I haven't paid too much attention to this but shouldn't it be a square of four pixels? That would be true double resolution, right?

Or is there some pixel trickery like those olpc displays? [http://wiki.laptop.org/go/Display]

px is a measure of length, not area.

A 12 px high font is 24 px high on a retina display.

A 12 px by 12 px box is 144 px^2, or 12 x 12 x 2 x 2 = 576 px^2 on retina.