My Nexus S is running the stock "soju" build of ICS from Google (build IMM76D). However, I have not seen the OTA update for Jelly Bean. Am I running the wrong version? Do I need to change some settings? Or should I expect the device to see the update soon?
You can try forcing your device to ask for a free download slot:
After this procedure the last checked date under Settings > About Phone > System Update should have been reset to 1970 or so. Tap the Check now button there and see if the update notification pops up now.
If this fails even after multiple tries, you can follow a slightly longer procedure described in this post, which involves taking out the SIM card.
Apart from that, you could just wait (you'll definitely get the update with your current config) or flash it manually.
My hunch is that you will indeed see the update eventually; I think the schedule depends on your carrier.
If you're impatient, you can download it directly from http://android.clients.google.com/packages/ota/google_crespo/9ZGgDXDi.zip, although I confess I was never able to get that to apply; you might also try downloading the entire 4.1 image and running the "flash-all" script; that worked for me on previous releases and thus I assume it'll work for you: https://dl.google.com/dl/android/aosp/soju-jro03e-factory-c9f5a67d.tgz