The focus steps for camera are somewhere along the lines of 50 cm, 1 m, and >5m. At this kind of precision, the depth map is basically 2 cubes.

It could be usable for closer imaging, like macro, but frankly it's not all that useful to depth-map a pen or a box of matches, save for a proof of concept.

And each-step capture with postprocessing is a good idea in theory but it will do you no good if the camera steps in what is basically "here, there and everything else".

I don't think onboard camera will produce anything useful by its own. You can glue a laser pointer to it and make sure it has a divergent beam. Measuring the size of the dot and the intensity could provide with a crude depth perception but it will have high resolution.

It could be done with additional hardware, say, via interference or by using an emitter-receiver and linking it to the device.

The focus mechanism is basically locked on to "end of line" for anything over 1 meter away, sometimes even less. For example, locking on to an image that is 60 cm away gives 2-3/5 a 60 cm, and 4/5 something in the order of 40-80. At this kind of precision, your map will be unreliable beyond help.

You could use 2 N900s. A stereoscopic image from 2 shots could determine depth to an amazing degree. If you know the resolution and distance between cameras you are basically limited by the quality of the shot alone. You could have precision as high as one meter up to -say- 30 meters away. A true depth map.