There was a restriction on VMs, but they've since eased up on that. I imagine it was because of the Hackintosh thing.
I'm reasonably confident the OS didn't actively try to detect that it was running in a VM, but VMware Fusion (and presumably other such software) would prevent you from installing the OS.
There was a relatively simple work around (a hack) that allowed you to install in a VM. I think it was as simple as touching the right path.