*Update: The AutoCAD DWG/DXF is NOT compatible with Google SketchUP 8. The only way as of now is to purchase Google SketchUp Pro, this comes with the DWG functionality built in.
With demand for this being high Google has created a plugin to bring in AutoCAD dwg files into SketchUp. This option was not available for the free version until September of 2009. Once installed the plugin in works great and you can import your AutoCAD .dwg file by going to File>Import.
There are many techniques to use while importing. Once imported into your SketchUp model it is ideal to place the AutoCAD portion on its own layer. It is very tough to just start building your 3D model by just using the CAD lines as SketchUp will not identify it as a solid surface if they are not connected (this is the case more times than not with AutoCAD imports). So it is best to use it as a reference and redraw the plan on top of the floor plan. A great tutorial on this can be watched below.
Overall, the AutoCAD import plugin by Google SketchUp works really well and is a MUST for professionals or not using AutoCAD files. At the time of this review this plugin is free and is available at sketchup.google.com. Though Google, has stated that this option will not be free for future versions of SketchUp. I give this plugin 4 out 5 stars, it is simply a must!