As Google has released Android 2.3.4 update for Nexus S mobile devices, now if you want to get root access to your device then following guide will help you to do that.
Download SU rooting file
Attach your Nexus S to your system with USB and copy the downloaded SU file to your phone’s directory
Hold down Power + Volume down buttons to restart your phone into recovery mode
When your device will be in recovery mode, select the mounts and storage from Mount/System and Mount/data
Navigate to main menu, from SD card option find Install Zip file
Select your copied SU file and select Ok
Flashing process will start, on completion just restart your mobile
Done! your Nexus S on Android 2.3.4 is rooted now.