There was a recent post from the famous Scott Drummonds about Running Apple OSX Lion on vSphere 5 and Scott provided his interpretation/opinion of Apple's EULA on virtualizing Apple OSX. Though the EULA can be somewhat confusing, it is true that with the release of vSphere 5, you now can run OSX 10.7 (Lion), 10.6 (Snow Leopard) and 10.5 (Leopard) as a supported guestOS in ESXi 5.
UPDATE: As of vSphere 5.1, the Apple Mac Pro is now fully supported on running on ESXi 5.1, to get more details, please take a look at this article.
The caveat is that in allowing VMware to run OSX as a virtual machine on vSphere 5, the physical hardware that ESXi 5 is running on MUST be Apple hardware and specifically the XServe 3.1. For those of you who do not follow Apple's hardware closely, the XServe line was recently EOL as of January 31, 2011 and that brings up an interesting problem. If you wanted to virtualize Apple OSX, you would have had to have purchased XServes prior to January 31st or start looking on Ebay with your corporate card ;)
Now, the Apple EULA is not the only thing that is regulating this requirement, in addition, VMware had to implement a software check within ESXi 5 to ensure that the physical hardware is in fact Apple hardware before allowing you to properly boot up an OSX virtual machine. The check looks for the SMC (System Management Controller) when an OSX virtual machine is being powered on and if this check fails, you will get an error and the virtual machine will be powered off automatically. The presence of the SMC is a new property that is exposed in the vSphere 5 API under "hardware" section of the ESXi host.
vSphere MOB and pointing your browser to the following URL:
Now you can easily determine whether or not your physical host can support running Apple OSX VMs.
As I understand from the beta, only XServer 3.1 will be officially supported and you will not be able to install ESXi 5 on older versions of the hardware. I have also heard mixed results on folks being able to install ESXi 5 on Mac Mini's and Mac Pro's. At this point, hopefully Apple has a change of heart and will update their EULA to allow ESXi 5 to run on "currently available" Apple hardware such as Mac Mini and Mac Pro.