Google put down Android 4.0 update for Nexus S because of some software problems.
By google:
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.
