I've been able to run windows 8 Developer Preview both on a vSphere 4.1 and 5.
This method is a bit special, but it works.
As I said before, it near the same than describe above, but using only vmware products (not xenserver).
You can read the article on my blog: http://www.sqaopen.net/blog/en/?p=9
If you have a better way, I need to know !