Good news for Nexus S owners – your OTA update to 4.1.1 Jelly Bean is ready and at least one user has reported it rolling out to his i9023 already.
The 114MB update, which brings the Nexus S' build up to JRO03E, is also available to download directly from Google's server for users with either the i9023 or i9020T hardware variants. For those wondering, this update shouldn't wipe data during installation.
assert(file_getprop("/system/build.prop", "ro.build.fingerprint") == "google/soju/crespo:4.0.4/IMM76D/299849:user/release-keys" ||
file_getprop("/system/build.prop", "ro.build.fingerprint") == "google/soju/crespo:4.1.1/JRO03E/403059:user/release-keys");
Those who don't want to wait for an OTA notification to hit their device can download the update from the link below. To flash, just make sure you're running a custom recovery (like ClockworkMod) and a stock ICS ROM (the update assert looks for IMM76D), copy the zip to your device, and flash as normal.
UPDATE: The official OTA for the i9020A variant has also been posted. Check here for the download.