Are you wondering about cloning an ESX host into a VM?
For Windows hosts there is software available to convert from virtual to physical hosts, but for ESX you'd be better off with a fresh install.
No, I'm wondering about arbitrary boot a single ESX installation as I may do with my Windows (VL) systems.
All I need with windows is installing a VM on a physical partition, switching ATA controller driver to standard, booting physical, then, after PnP has done its work, cloning the system to any storage controller that is present. As a result such a guest may boot successfully on virtual or on real hardware.