VirtualGeek has a post about VMware used in a Sharepoint scenario with a 74% energy savings.

The results:
In terms of performance, (omitting the SQL backend - which has been virtualized in other tests showing EXCELLENT performance), across 3 baseline tests, on average:
Our Virtualized SharePoint server infrastructure farm out-performed the physical SharePoint farm by 4%,
But only used 26% of the electrical power (watts) required to power the physical server infrastructure - put another way, that’s a 74% power saving over physical, put yet another way, going physical means 380% more power.
1017 Watts versus 3952 Watts. 6 Power cords versus 22

As Microsoft gets ready for its Hyper-V launch, we’ll see how their #’s match up to VMware’s.

Here is one sharepoint on Hyper-V blog entry. It is a demo environment, but they show a 100% performance improvement for Hyper-V vs. Virtual Server 2005 R2.