Ok my Nexus S runs on 4.0.4 ICS, build IMM76D, rooted many time ago and all works until...
All is ok, but today installed Titanium backup and Superuser denied access to it... intall then Busy Box, but.. denied...
then i install Root Checker, response: Rooted! but above screen a fast sharp write.. superuser denied access to Root Checker LOL

Help please XD