Eric Gray, an employee of VMware, has a great site on VMware solutions titled vCritical.com. Today he posted a great posting on vSphere 5 describing how vSphere ESXi can run as a virtual machine on a vSphere 5 ESXi host. Also, nested 64 bit guest can be run as well. So image a phyiscal server running vSphere 5 EXSi. Install two ESXi machines as guest. An install Windows Server 2008 R2 as guest in those guest!
Great for demo, study etc purposes. Also Hyper-V can run as a guest on vSphere 5.
More info here as well and here!