No face unlock for the Nexus S?
That's correct: starting Ice Cream Sandwich (ICS) 4.0.1 and until the curent (at the time of writing) 4.0.3 the Face Unlock feature is not available to any version of Nexus S (NS).

This thread on xda-dev provides various speculations as to why Face Unlock is not available on NS:


lower quality front camera hardware on the NS
the vulnerabilities of this feature
(check out the note on the setting that says that Face Unlock is low-security and experimental)
having this feature exclusively available for Galaxy Nexus


Bottom line: Face Unlock is present in various ported ICS ROMs, but it's reported to be pretty flimsy at the moment.