Google halts Nexus S ICS update in certain regions

The Android 4.0.3 update for the GSM/UMTS version of the Nexus S began a few days ago and by now several of them are running the latest version of Android. However, there are still many left who aren't and it seems it will be a while before they get it.

Certain users are reporting that they are getting the update notification on their phones but when they begin the update process the download fails to start, after which you have no choice but to cancel the process.
When this problem was posted on Google's forum, Google responded by saying that it has paused the update in certain locations and is monitoring feedback from other regions.
This may have something to do with the battery life issue being reported by many Nexus S users who have upgraded to 4.03 and something we ourselves have seen on our Nexus S. It seems Google may be in the processing of solving this problem and will then issue a patched version to everyone.
Till then you will have to wait for a while to get the update automatically. Or you can always download the ROM and update manually.