This application might be what you are looking for:
http://www.world-machine.com/
It has the qapability to construct realistic terain and can export terrain in various formats.
Bryce is in my opinion one of the easiest terrain editors to use, but takes forever to constructs large areas (I'm working on a Greayhawk . Part two of my Bryce tutorial will cover terrain construction in more detail. Hopefully it will be available in this forum in a week or two.
//Anna