Few minutes ago, Google has finally rolled out Android 4.0 Ice Cream Sandwich for the Nexus S.
The update will be available via OTA, but if you can’t wait, follow this guide! 
- Download the OTA Package from here: http://t.co/i8NjJoLo
- Rename it to update.zip
- Copy it to the root of the internal storage
- Power off the device, hold Volume Up and Power to reboot into the bootloader
- Select “recovery,” then hit the power button
- Once the warning triangle appears, hold the power button and hit volume up
- Select “apply update from /sdcard” and choose the update.zip
- Once the process is finished, simply choose “reboot system now”
- Once it reboots, BAM! You’ve got ICS. Congratulations!