> Supposedly the key to this is the fact that intensity is proportional to 1/distance^2, but just how they use this (given that different objects will be different in brightness) the article doesn't say.

I'd imagine something not far away from :

pixel(x, y) = max(intensity(x, y, ccds))

The IR distance system has been around for a while. Sony released a camera which recorded the distance per pixel too, they introduced it as an alternative to green screening, using the distance map as a matte. I can't find what that's called atm. and though I looked forward to using one, I've never come across one in my film work.