The Terragen importer sounds great, I've seen a number of models that have been created with it that look great, including height fields converted from Mars MOLA (Mars Orbiter Laser Altimeter) data from NASA. Really opens up some possibilities for creating realistic mountains and plains in scenes, with spacecraft models where you may not want to involve importing the models into Bryce.
One of the reasons I bought AC3D for its file import (like LWO files), a product strength that I look forward to seeing it expanded!
Can't wait...
Steve