Once the cost goes down, multiple LIDAR units can be mounted behind the windshield. Cars designed for automatic driving should reserve the top inch or so of the windshield for sensors, with the optical sensor suite mounted at the front edge of the roof and covered with padding matched to the headliner. (Yes, auto glass today is often infrared-opaque to reduce heat input, but the IR-opaque layer could be omitted for the top inch.) Putting cameras behind the rear view mirror is already done, but more space may be needed.

The rotating Velodyne thing Google uses is an interim research tool. In production, expect to see better design.

With enough LIDAR units, full-circle coverage can be achieved. This is already being done with cameras. The rear and side units need less power and range than the front-facing units, so they can have a smaller collecting aperture, making them easier to place. For the front-facing units, the emitter can be spread out, allowing higher power. The rules on eye safety are concerned with the maximum power coming through a 1/4" hole (human eye pupil). If the laser emitters are spread out over the width of the windshield, the maximum power per unit area is reduced.

The CMU/Cadillac demo a few years ago managed to conceal all the sensors. It can be done. It just costs too much until one of the three companies claiming to be building low-cost automotive LIDARs Real Soon Now manages to deliver.