Notch, the guy behind Minecraft, also roasted Unlimited Detail. His takeaway point was "be excited about voxels, but not about UD." Then he pointed to somebody doing the voxel thing much cooler than the UD guys:http://atomontage.com/
He's got procedural generation of the landscape, and it looks awesome. He's actually more interested in physical simulation of his atoms than on the convenient side effect of it looking cool. His stuff isn't copy-paste repeats. It's all deformable/destroyable. And if you find the right demo videos, you can see that he can do very large areas -- possibly not unlimited, but "past the horizon" at least. He's already got it interactive where he can drive around in his landscapes leaving ruts.
His smallest voxels are larger than the UD guys, but he could always procedurally fill in around them in the necessarily small viewing area you would have if you got down close enough to see. But if you're walking along 5-6 feet above ground level, you're not going to notice anything but awesome ground.
Now, everything he's doing so far as I've seen is different kinds of rock and dirt, and "concrete" buildings. Gotta get vegetation and water in there somehow. But I still think his approach is much cooler than UD.