"Longmode is unsupported. It is required for 64-bit guest OS support. On Intel systems, longmode requires VT-x to be enabled in the BIOS. On nested virtual ESX hosts, longmode requires the "Virtualized Hardware Virtualization" flag to be enabled on the outer VM."
I seem to remember that in version 5.0 you had to configure a given parameter in the ESXi console. For ESXi 5.1 this has changed according to this VMware KB.
It states the following:
"Virtualized HV is fully supported for virtual hardware version 9 VMs on hosts that support Intel VT-x and EPT or AMD-V and RVI. To enable virtualized HV, use the web client and navigate to the processor settings screen. Check the box next to "Expose hardware-assisted virtualization to the guest operating system." This setting is not available under the traditional C# client."
So, access the web client, locate the VM, right click -> Edit settings, and check the box as mentioned (for the parent VM, not the virtual ESXi...). It works like a charm, see screendump below: