If you miss it the facelock feature from ICS 4.0.3 (NS-NS4G), flash this file and try.
1. copy FaceLock (4.0.2 - 235179).zip to 'USB Storage'
2. reboot recovery and install
App version: 4.0.2 - 235179, shows 4.0.3 in applications.
Tested, and working, but slow, and the face recognition sometimes is not perfect.
(probably because of the nexus s vga camera)