There was an interesting question that was asked the other day about changing the hardware serial number for an Apple Mac OS X guest as the generated serial number is not compatible with services such as Apple Caching Service or iMessage. I recall seeing this question get asked awhile back, but I could not immediately find the answer but thanks to Darius Davis (VMware Engineer) who provided the quick answer.

We have a facility to generate a "short" serial number which should be suitable for recent Apple software.  The option is enabled by default for OS X 10.9 guests.  To enable it for earlier guest OS versions, you'll need to power off your virtual machine and edit its configuration to add the following option:

SMBIOS.use12CharSerialNumber = "TRUE"

As mentioned by Darius, if you are running Mac OS X VM prior to 10.9 (Mavericks) you will need to add the following advanced VM setting by first powering it off and then add the above setting. There are two recommended ways of performing this change using either the vSphere C# Client or vSphere Web Client and instructions are listed below.

Note: Though you can also edit the VMX configuration file by hand, for those that are not familiar on how to reload the configuration file, it is best you use the UI.

vSphere C# Client:
Edit Settings -> VM Options -> Advanced -> Edit Configurations

vSphere Web Client:
Edit Settings -> Options -> Advanced -> General -> Configuration Parameters

Once you have added the advanced setting, you can now power back on your Mac OS X VM and when you click on the "About this Mac" option on the upper left hand side of the Apple icon you should see the Mac OS X version string. Click on the version string twice and you should now see the serial number that is generated which should not be longer than 12 characters.

As of writing this article the latest Mac OS X 10.9 (Mavericks) is not yet officially on the VMware HCL for latest release of ESXi 5.5 as the OS just came out recently, however it is still possible to create the a Mac OS X 10.9 guest using the new vSphere Web Client. Be sure to keep your eyes on the VMware HCL for support of Mac OS X 10.9 on ESXi 5.5 here.