"A quick note just to clarify, there are no 5.1.0a and 5.1.0b versions available for ESXi"
Actually ESXi 5.1.0a does exist: VMware-VMvisor-Installer-201210001-838463.x86_64.iso
https://my.vmware.com/web/vmware/details?productId=285&downloadGroup=VCL-VSP510-ESXI-510-EN