(From first page)
http://sketchucation.com/forums/viewtopic.php?f=15&t=44282#p400386
@unknownuser said:
.../
You probably need to use some external software (I have seen one some time ago, but I don't remember the name. It has export capability to Sketchup, can handle crests and is fully specialized on this problem of terrain shaping)./...
Hi Fredo 
your Toposhaper looks very promizing

You were probably talking about Terragen
( Terragen 2.4 Free Version Now Available)
Here a model made by Google Guy (G.G.):
http://sketchup.google.com/3dwarehouse/details?mid=f4350227d1c9debd6623100010a93a06
some really nice models by TaffGoch:
http://sketchup.google.com/3dwarehouse/search?tags=Terragen

Here from [url=http://wetbanana.deviantart.com/:6upgx29j]Wetbanana terragen gallery[/url:6upgx29j]

picked up into Terragen forums:
[url=http://www.planetside.co.uk/forums/index.php?topic=5763.msg59956#msg59956:6upgx29j]Re: Terrains For Google SketchUp?[/url:6upgx29j]
@unknownuser said:
For minimal effort your best bet would be to forget all about generating your own terrains and import real world elevation data instead. If I recall correctly you can directly import .dem files into sketchup. Try downloading some files from here and see if they work out for you.
[url:6upgx29j]http://seamless.usgs.gov[/url:6upgx29j]
The link appears to don't work 
but this one is ok (free topo download in pdf)
Illustrates available US Topos
[url:6upgx29j]http://viewer.nationalmap.gov/viewer/[/url:6upgx29j]
Historical Topographic Map Collection
http://nationalmap.gov/historical/
etc..
++simon.