There's been a lot of hype/talk about Windows 8 and if you wanted to test drive the new OS, you might consider using the latest release of VMware Fusion 4.0.1 or VMware Workstation 8 as Windows 8 is an officially supported guestOS. Though what if you wanted to run it in your vSphere 5 environment? Well you can with a small hack.
Even though it's not listed as a supported guestOS, you can manually tweak the .vmx configuration to get ESXi 5 host to recognize the guestOS type. You just need to create a generic Windows 2008 system and then from the commandline or by exporting the .vmx using the datastore browser and then edit the configuration file. You will need to make the following change to the guestOS paramater:
guestOS = "windows8srv-32"
guestOS = "windows8srv-64"
One you have made this change, you will need to re-register the virtual machine or reload the configuration using vim-cmd vmsvc/reload operation.
Another method just using the vSphere Client without any modifications to the .vmx is to just create a virtual machine and select any guestOS type. Once the virtual machine has been created, there is actually an option in the guestOS to select Windows 8 32 or 64bit that can be selected. If you wish to automate through the commandline, then you can use the method above or you can just use the vSphere Client.
Note: This is not officially supported from VMware of course, use at your own risk.
UPDATE1: It looks like when Windows 8 64bit is booting up for installation, the virtual machine core dumps with the following error:
vcpu-0| MONITOR PANIC: vcpu-0:NOT_IMPLEMENTED vmcore/vmm/intr/apic.c:1804
Something similar occurs with Windows 8 32bit that gets past the panic but an error message is thrown on the screen regarding HAL initialization failure. Currently there are no workarounds and I've reached out to some of the folks at VMware to see if there's any tweaks that can be made to support this. As I mentioned earlier, this is an unsupported OS/hack, so it may not work at all. Sorry to get everyone's hope up, the new Fusion 4.01 and Workstation 8 might still be your best bet to test out the new Windows 8.
UPDATE2: VMware has released a KB article http://kb.vmware.com/kb/2006859 regarding Windows 8 and vSphere 5 support. You can subscribe to the KB article for the latest update on running Windows 8 on ESXi 5.
UPDATE3: I recently saw a tweet by Raphael Schitz and it looks like you actually CAN run Windows 8 on ESXi 5. Raphael was able to run Windows 8 by first running Xenserver as a virtual machine and then creating a Windows 8 VM that would run as a nested guestOS within Xenserver virtual machine (pESXi 5 -> Xenserver VM -> Windows 8 VM).
Note: You may need to reboot the system one additional time if it does not automatically load.
Here is a screen shot of Windows 8 64bit running on the latest release of Xenserver 6 running on ESXi 5: