Are you sure it's an update notification? I received another message about ICS a few days ago that was similar to the message that went out just before and a shortly after the ICS update. And just like those messages, it looks like an update notification, but it's not.
When I rebooted last night, I got some kind of message about it doing a system update. It only lasted for about 15 seconds - went through a "system update" and an "app update." Odd, since I got the OTA a few weeks ago.