Google hosts their flagship device OTAs online, fortunately, under the codename crespo for the Nexus S. Some searching along those line resulted in this Android Police post (affiliate links removed):
Well, that didn't take long. Here are the instructions to [get] your Nexus S rocking ICS right now. Do not try this on the CDMA Nexus S!
- Download the OTA Package
- 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!
The link there is
http://android.clients.google.com/packages/ota/google_crespo/VQ8PQk_V.zip. Koush notes that this is for the T-Mobile version, the i9020T. From dottech.org:
There are unofficial reports that the update works for i9023 (the Super LCD version of Nexus S); and can work for i9020A (the AT&T/Telus/Koodo Nexus S) by flashing compatible radio files. If you are a brazen fellow you can attempt to update your i9023/i9020A with the T-Mobile ICS package but be warned: You might get a brick.