Usual disclaimers etc, this worked for me but no promises The Nexus (and most other Samsungs) are almost impossible to brick. Almost…

I had some trouble doing this, so a quick rundown of how I got it on in the end. Keep in mind mine was rooted and running a custom ROM. I kept getting build.prop errors, and nothing would work.

Official ICS for GSM phones is here. (If you are already on 2.3.6, if not, see below)

(UPDATE: This only works for 2.3.6. Am trying to find link for the full ROM which is 161MB the one linked to is only 128MB) this guide will still work but you can go from 2.3.1 straight to ICS if you find the 161mb file.

Update 2. Full file for going from 2.3.1 is here

1. Make sure you are running stock 2.3.6, you can either do this from Odin, which is what I did. I got Stock from here and flashed it in Odin by unpacking the .rar file and putting the 3 .tar  files in the relevant sections in Odin. Do not need to repartition or use a .pit file, just bootloader, PDA and CSC. If you need to do this you get to download mode on Nexus S by holding both Vol up and Vol down and then powering on.

2. Once it boots up you are on stock 2.3.1 and if you sign into Google, in a few mins, you get the OTA to 2.3.6

If you are already on stock 2.3.6 it is probable that you can start from step 3

3. Take the ICS file you download already and rename it as update.zip and copy it to root of Nexus S SD card (internal only of course)

4. Boot into Recovery by turning off phone or pulling battery, and hold down Vol Up and then Power. Once in recovery, wipe data and cache options and then select “update from SD card”. You should see the update.zip then (if not you put it in a folder by mistake) and just select it and wait.

If you boot into Recovery and get the Android with a Triangle and Exclaimation mark, do not panic!, just hold volume up and press power, and you get Recovery.

As a final step, I formatted the USB storage, which of course zaps all your data and apps, so use with care. ICS re installs all your Apps later though.

Oh, and its MUCH nicer on a device with buttons!