I'm sure I'm not alone in wanting to create flat areas on terrains that are the right size to put a building or other object on.  I've released a new free python script (usable in Vue 6 Infinite and Vue 6 xStream)  that will create a flat area on a terrain under an object.  It's a little rough at the edges at the moment: it copes with terrains that have been scaled but not that have been rotated. If it proves useful I'll come back to it later.

Find out more about the script (and download it) from by blog: Terrain Matching Script

If anyone who uses it makes a better picture to show it off than the quick one I've put up I'd be interested in seeing the results.

Yes, I've had that need, from time to time. I normally do it in two ways: either I do a boolean subtraction of a cube from the terrain (I previously scale the cube to match whatever I want to put on top), which sometimes causes crashes in render time if I dont bake the result, or, if I want to put the object higher than the terrain, I put a "dummy" rock textured cube, like if it was built there (not very nice and a bit artificial, I have to admit...).

So, I salute your script with great joy! :-) I will give it a try! Thanks for sharing it with the community!