Many GSM Samsung Nexus S owners have been patiently waiting to see when Google would push the next update to their devices, and the answer is right now. Android 4.0.4 is currently rolling out to users showing a build of IMM76D. You have a few options when it comes to installing it of course. If you're on a stock device running Android 4.0.3, you can download and install directly from the OTA or use the directions below:
Rename it to update.zip. This isn't needed for the Nexus S bootloader, but it makes things easy, if that's how you prefer them.
Copy it over to the internal storage on your Nexus S.
Power off, then hold volume up and power to reboot to the bootloader.
Using the volume key to navigate, select recovery, then use the power button to confirm
When you see the warning triangle and arrow, hold the power button and tap volume up. You'll see a menu.
From the menu, select "apply update from /sdcard", and choose update.zip from the list.
Let it do its thing and update your system, radio, and other partitions. When finished, choose "reboot system now"
If you're rooted, there is a Clockwork Mod flashable zip file available for download as well, but again you will have to be on Android 4.0.3 to make use of it as it is not a full OS update. The CWM flashable file is stock, with a insecure boot image with Busybox and Superuser installed, as well as the stock recovery removed.