[windsurfer]

I bet they would work great. If you could tell a 3D artist that he can use as much detail as he wants for this game... well, you'll get what they show on Zbrush central. All those models have hundreds of times more polygons than what would ever be accepted in a real game.

[dirtbox]

I use ZBrush to create displacement maps using HD geometry, which doesn't use actual polygons in the classic sense. A lot of the ultra high poly models you see on the turntables and such use the same technique. You can easily make a mesh well over a billion polys that doesn't impact the system. http://www.pixologic.com/docs/index.php/HD_Geometry

I doubt they'd have anything anywhere near ZBrush's flexibility and depth.

[windsurfer]

You could totally take a displacement map and tessellate a model with it. In fact, that's what OpenGL4 and DirectX does in hardware.