My env: 8-core 3Ghz Mac Pro w/ 10GB RAM.
I built a VM yesterday using RHEL 4 update 4, allocating 2 CPUs and 4GB RAM.
As you can see in the attached pics, with 2 CPU installed, OS/X reports a usage ranging between 40-55%, while the guest is reporting 0.5-1% utilization.
When only changing the number of CPUs assigned to the VM to 1, that drops to 12-18% on the host while maintaining the utilization on the guest.
Many of the conversations around the high idle CPU usage in this forum have been in environments on dual core boxen. Being an 8 core box, there should be plenty available for use by the OS.
I haven't had an opportunity to place the VM under load to model the performance, hopefully the apparent idle usage degradation does not appear under load.