There are 3D GPU rendering systems, like Furryball.
Generally, GPU has been for previewing and CPU for rendering. But this changed in Ae CS6. If you use the built in 3D, and change your comp settings to ray-traced, then the GPU will be used for all of the ray-tracing operations, both in previewing and rendering. GPU is also used for accelerating certain effects, such as cartoon (for preview and render).
I believe this is also true of plug-ins like Element 3D.
I think a good GPU is becoming ever more important, and I guess as they improve, that more and more stuff will be being done on the GPU.