Check out LightUp.co.uk which is a non-free visualization and lighting plugin for SketchUp (free and pro). I have used the free demo version and could export my SketchUp scene into Unity, including shadows. I've made some short screencast movies on Vimeo on how to export a lightmapped scene from SketchUp 8 into Unity 3: http://vimeo.com/19204786
Other solutions: the dreaded export to Google Earth and into Collada, which did work on my Mac, but not on a PC I tried.
Or use intermediate conversion software (Blender, FBX Convertor, Cinema4D (certainly not free) or others).