Apparently the update is continuing to download for some users, but in some cases it won't install, though Wilcox says that this is "a different scenario." We've reached out to Mountain View for an update, but let us know about your experience in the comments.
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.
Samsung Nexus S