Lately I've been quite involved into a virtualization project: running Mac OS X and Windows as workstations on top of VMware "bare-metal" hypervisor ESXi on my Mac Pro. It requires a good knowledge of virtualization and VMware products like ESXi and VSphere, serious sysadmin skills, and lots of perseverance.
I've finally managed to boot a Mac OS X 10.6.8 virtual machine on top of ESXi, on my Mac Pro with a proper ammount of RAM (12 GB), and the graphics card in passthrough mode. That required a manual tweak of the vmx file.
The VM wouldn't boot when configured with both the graphics card in passthrough and more than 2 GB RAM. I've had to add into the vmx file those two lines:
pciHole.start = "1200" pciHole.end = "2200"
Then I was able to boot my Mac OS X VM with 12 GB RAM and the graphics card in passthrough. Great. I'm still lacking passthrough for USB keyboard and mouse, meaning I need a remote computer with VSphere Client to control my VM using the embedded console. But the VM uses the physical ATI Radeon, and the physical screen, and in theory it could use full GPU power.
It looks like things are working OK, but it'll take time and many more tests to make sure everything is really working. For example, I was not able to launch 3D FPS games like Left 4 Dead and Left 4 Dead 2 into the VM. The game would crash on launch.