I was looking around on the SketchUcation forums and found this link to an amazing set of plugins for SketchUp created by Vali Architects. They are geared toward terrain modeling and roof creation. They aren't free, but very affordable ($15-$25 each). Definitely worth checking out if you're using SketchUp for architectural, civil or landscape work. I've been wishing for something like this for a long time, and now in the past week I've found tools like these for both Revit and SketchUp.
The Revit tool I found is called Siteworks by Eagle Point Software. It will cut parking lots, sidewalks, curbs & gutters, retaining walls, berms, swales and other things into Toposurfaces. It is full of features and therefor obviously costs a lot more. Watch the demo videos on their site to get a better idea of what it can do.