Since the last update this is a piece of cake.
Say you have your scene set up the way you like. To make rendertimes as short as possible during exporting the terrain temporarily disable athmosphere, clouds and GI, uncheck "cast shadows" of the sun. If you want nothing but the terrain, uncheck all objects too.
Go to the render tab, under "sequence/Output" go to the Micro exporter, check it and hit "create new micro handler". Important: choose "Micro exporter", not LWO micro exporter!
Go to this micro exporter, set your farthest distance (it's set to a very high value by default). You have to test that. You can start with a value of 10000. That's going to be how far away the border of your exported terrain will be.
Under filename write "your filename.obj". Hit Render. Done.
This way you have the portion of your terrain your camera can see.
