Altitude is calculated from the planet surface unless you have "Use Y for altitude" checked in your shader, but that's only really useful for loaded terrains that also have "Flatten first" checked in their displacement, which in turn isn't practical for a whole planet as you get a flat terrain, not a spherical one.
As for placing terrains, I don't think you can place a terrain on the other side of a planet. Terrains are currently mapped by simply projecting the terrain down onto a sphere from the top. To cover large sections you need to select an appropriate projection for the terrain. It would be nice to be able to place equirectangular projected terrains onto portions of a planet, but this is not possible at present. You can use a spherical projected image map for a displacement shader for the entire planet but this limists your resolution.
I've been setting up multiple terrains of different resolutions to provide padding for a high resolution area of interest to allow the camera to fly down from a very high altitude (
http://forums.planetside.co.uk/index.php?topic=2036.0), but this is still restricted to working on one hemisphere.
On idea I had toyed with was using transform shaders to move the terrain and clouds around the camera. The camera would stay at 0,0 changing only altitude and orientation but I don't think that will help with loaded terrains as it still doesn't address projection issues.
