Missing Face Unlock from your official ICS ROM on your Nexus S, time to shift to a custom AOSP based firmware. It is practically stock apart from few tweaks and Face Unlock support. Released by devs at AC Forums recently for both Nexus S GSM and Nexus S 4G versions, it can be downloaded now.
Installation process is pretty simple, you will need ClockworkMOD recovery on your Nexus S and rest is just some easy flashing. You can find the download links and full install instructions here (for I9020T) and here (for NS-4G).
PS: Only flash this if you really want Face Unlock, otherwise there is no other incentive for you.