I've been building a new machine and I have put Win2K8 Enterprise on it. While I have no specific reason to enable the hypervisor other than my absolute, unbounded enjoyment at saying the word "hypervisor", I also was going to try to run a few of the server-like programs that I use at the house (for example, my music server software).
So, I turned it on and found that it would randomly reboot 1-3 times/day. (I realized this because my Squeezebox would stop showing me the time downstairs, which means the server went away). While I can (and did) put in the registry keys to auto-login to my server and get the music player to work more reliably, I was disappointed because this hardware was a combination of pieces I had been using successfully in another machine.
My best guess is that I have some sort of driver issue with my motherboard/cpu/etc and win2k8. In fairness, the system is about 2 years old, and I doubt they did extensive testing on the final version of win2k8. I will go hunt for updated BIOS and drivers, but often this means you need to upgrade to a more recent motherboard...
FWIW, I am running a core 2 duo w/8GB RAM on an Asus P5B-E. The video card is an ATI Radeon x2600 PCI-E, I believe. Not much else in there but drives.
Based on my searches, not too many people are running such a combination :). Oh well.