When I lock my Nexus S, the screen flashes once:
It turns black, then the home screen appears again very shortly, then it's black again.
Do you have this aswell with a Nexus S on ICS 4.0.3? Is there a fix?
It's a software issue with Ice Cream Sandwich as far as I know, so no fix. It may only be present on certain models however, I only have anecdotal evidence for this but I have the issue on my Nexus S model IML74K but know someone with a different model of Nexus S and they don't seem to have the issue...