Nexus S might soon be getting the Android 4.0 ICS treatment from Google as tweeted by one of the Google employee that he has received the firmware update, Google might be testing the Android 4.0 update internally before rolling out to the end users but there is no official word about it yet. The tweets were also removed later but you can see the capture below.
It was an Over the Air update and may become available soon for everyone. Check this page later for official update on the Android 4.0 for Nexus S.
Download the early build release of Android 4.0 ICS for Nexus S