Infra-red cameras are used in the LeapMotion controllers. I don't think it is anything special about the spectrum however, coupled with Infra-red lights it's an easy way to cut down on the noise in the comtrollers field of view.
Depends but there are computer vision techniques for deriving depth from two or more calibrated cameras. It may not be super accurate but it is likely do-able.
Are you thinking of some active scanning technology such as LIDAR? Or the infrared grid-of-dots projection that the kinect uses?