We're having a discussion here about Hyper-V and vmware. Am I right in thinking that ESX and Hyper-V are competing technologies and it is not possible or desirable to run hyper-v on a box virtualized on ESX? Is this because the machines vmware emulates do not have virtualized implementations of the process virtualization support?
Yes, Hyper-V and VMware ESX are competing technologies. I don't think there's anything preventing you from running one on top of the other (you can run VMware on a machine that does not support hardware virtualization), but I don't know why you would want to.
Windows 2008 will not let you add the hyper-V role to a machine running in ESXI.
Yes, running a virtual machine (such as Hyper-V on Win2k8) as a guest vm inside VMWare ESX host is not a good idea. Not (only) because they're competitors, but technically there's a lot of work the processors do behind the scenes using nested page tables. To cut to the chase, an article (link) briefly explains the benefits:
Creating a virtual machine that hosts another virtual machine I think would be extremely detrimental to overall performance. Granted I think if you wanted to host Hyper-V within ESX, it should be feasible but I would argue it would be a waste of resources.
I am not sure whether or not this would work or not, but I can't think of a reason why anyone would want to. There would almost certainly be a negative impact running hyper-v on ESX as I am sure then virtual processors wouldn't provide any Intel/AMD VT support.