I’ve been playing around with the new vSphere 5 Hypervisor (ESXi5) lately and I came across a few cool tricks from virtuallyghetto (http://www.virtuallyghetto.com/2011/07/how-to-enable-support-for-nested-64bit.html): running x64 VMs on ESXi5 while itself is a VM under Workstation 8 and running VMs under Hyper-V server under ESXi.
Running a Hyper-V VM while Hyper-V is itself a VM under ESXi5 does work, it’s not really something you’d want to do, unless you’re masochistic. I was able to get Windows 2000 Pro
running walking as a VM successfully. The stability of ESXi seems to be greatly affected by doing this also, since my ESXi host hung at some point overnight.