I have a nexus s i9020a on Koodo in Ontario, I didn't get the OTA but I installed using the IEzMw3FU.zip file, just download it from the link
h t t p : / / v 1 9 . l s cache7.c.android.clients.google . c o m / packages/data/ota/google_crespoa/IEzMw3FU . z i p
(the spaces are because I'm a new user :P)
this is a link connected with google and is official! Also I know this is official because google has released factory images on code.google.com for the i9020a.
When you have the IEzMw3FU.zip rename it update.zip and place on your nexus s. Next reboot and hold the power and volume up. Use volume down to go to recovery and press power. When you get the triangle press power and volume up and go to install update from sd. Find your update.zip and click. It will update.
This only works for STOCK. This means NO modifications in any way, such as radio, etc. When it finishes you can reboot. It will make your apps ready for 4.0.4. And VOILA! You're now running ICS! I update from 2.3.6 stock. Never modded. Hope this helps!