Position your terrain in front of the camera so you can't see the front edge. Use the same texture on the terrain and the default plain. I used Graysand from the terrains - plains menu in Bryce 6 for this. Pick a good sky like this starfield and you are done.

If you want more background you can build more terrains or mountains. as long as you step down from the camera perspective you should be able to hide the edges.