It looks like Google has started rolling out first OTA (Over-The-Air) update for Google Nexus S users. The update is officially marked as Android 2.3.1 Build GRH78 and it is just 1.9MB in size.
Official change log is as follows:
This system update from Google contains important bug fixes plus the latest version of Maps.
So yea, apart from the usual bug fixes and the latest Google Maps 5.0 app for Android, there is nothing much to look for in this update.
If you haven’t got the OTA update notification yet, you can force your handset to get it by dialing *#*#checkin#*# from the phone dialer to force your phone to prompt you for this Android 2.3.1 update.