btw, I starting reading Google SketchUp for Site Design: A Guide to Modeling Site Plans, Terrain and Architecture (the name alone takes up the first 12 pages of the book! ) yesterday and it talks a lot about importing CAD drawings and gives some really helpful advice on a couple different methods to through the cleanup process, as well as some various ruby scripts which speed things up. He also has some interesting ways of using a few tools that help with site modeling as well. I've only skimmed it so far but if you work a lot with terrain you might check it out. There don't seem to be many SU books out there yet so getting detailed info can be tricky, at least that's my experience.
arklandesign started a thread on the book here...
http://forums.sketchucation.com/viewtopic.php?f=179&t=21371
-Brodie