By now, I think, a lot of people is having the same problem. Problem though it is, it's still fixable. I downgraded my Nexus S yesterday from ICS 4.0.3 to Gingerbread 2.3.6. The process took about 10 minutes, but the search of how to do it took a few hours. I have to say that I did not find any instruction particular for "How to downgrade from 4.0.3 to 2.3.6", all I could find is how to root a Nexus S or how to install superuser and stuff like that.
To downgrade from ICS 4.0.3 to Gingerbread 2.3.6 is quite simple, but it varies from user to user. The basic steps are: - get some files needed on your PC (fastboot, clockworkmod recovery image, usb_driver for windows and, of course, the proper ROM image for your device. - unlock bootloader (using fastboot from PC) - flash clockworkmod recovery image(using fastboot from PC) - copy the ROM image from PC to the device (using clockworkmod from device) - flash the device with the ROM image (using clockworkmod from device) - reboot - lock bootloader
I will try to post as fast as I can a set of thorough instructions of how you can downgrade.
While I was Google-ing, I found a lot of "how to root your nexus s" videos and instruction. The downgrade process is similar BUT not the same. The whole idea is to get your device back the way it was without messing up your warranty. Just a final note, to downgrade you do NOT need to install any super user app or anything like that.