Google has apparently stopped rolling-out Nexus S ICS update in some regions for the time being. This step is said to be normal procedure and company does it every time it rolls out an OTA to monitor the reaction from users with already updated smartphones.
So, if you have not got the OTA notification till now, don’t worry it will be available on your phone within this month, or if you got the notification some time back but you declined and now you are not able to get it again, same case – you will get it soon.
According to Google’s Paul Wilcox:
If you received an update notification a little while ago but the update isn’t currently available for your phone, this is likely the result of Google pausing the update in your area while we monitor feedback. The Android 4.0 update is continuing to roll out around the world so your phone will receive another update notification when it’s available again in your region.
PS: You can always install it manually using the guide given here.