What do you mean by 'ESX host serial number' ? ESXi License key ? ESXi hardware serial number (i.e. the hardware manufacturers serial number) ?
BTW you may want to ask this over in the PowerCLI forum.
Get ESXi host of a VM:
$ESXi = (get-vm vm).VMHost
Get license key of ESXi host:
$ESXi | Select-Object -Property Name,LicenseKey
The serial number of my ProLiant here can be found here:
$ESXi.ExtensionData.Hardware.SystemInfo.OtherIdentifyingInfo[3]
But I was told that this method to get the serial number is not reliable since different manufacturers may provide different info there.