Actually Jeremy Birn talks about creating shadow only lights in 'Digital Lighting and Rendering' - 'shadow only lights can be tremendously useful if you want to control the exact, size, angle and perspective of your shadow without changing the lighting of the scene'.

I haven't done this, but he says in maya, 'you can create a shadows-only light by sliding the light color down to pure black and setting the shadow color to a negative value'

Though, I agree with happymat, your scene will look perculiar with no obvious source of light.

Hope this helps.