As soon as Google purchased Sketchup it was obvious that a free version would soon emerge for the mass market (Windows only at the moment). From Google's perspective it is really useful for them to have their 'virtual world' (Google Earth/Maps) populated with user-created 3D models. The best tool for the average Joe when it comes to doing this is SketchUp. My guess is the ulitimate goal from Google's perspective will be to have elements SketchUp embedded within Google Earth so that general users can quickly and seamlessly add their own 3D data whilst using Google Earth. Whether or not this happens or whether users take up the challenge and recreate their 3D world within Google is uncertain but one thing that is for sure is that 'free' SketchUp will really shake-up the relatively stagnant CAD market space.
For too long the US$500 price range for sketching tools has been the norm within the sector. Google obviously does not care about taking down AutoDesk but in one move its become the dominant player in the low-end 3D modelling market.