If you don't need runtime terrain generation you should look at acropora from voxelogic. It can generate infinite procedural terrain and has a ton of filters and can export generic voxel data which you could then load into your game.
Here's some stuff I made in it the other day:
http://dl.dropbox.com/u/6281166/terrain_test01.png
http://dl.dropbox.com/u/6281166/terraintest3.jpg
http://dl.dropbox.com/u/6281166/terrain.png

@ape - Yeah, the Acropora modeller is cool. If you want to import them into PolyVox there is some small chance that the following code will work:
http://gitorious.org/thermite3d/thermite3d/blobs/master/Raw2Volume/source/main.cpp

@Shanee
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