No face unlock for the Nexus S? I seems to me, that the Nexus S did not get the face unlock feature with ICS?

Why would this not be supported for the Nexus S? Is it the hardware?

I wonder because the code is open source and I assume any other competitor would not restrict this feature just because it's not perfect yet. I mean: There are still quite a few other things that also do not work perfectly.
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.