The update will roll out in waves, as is very common, with GSM devices being the first in line. The entire roll-out should hopefully finish in under a month.
Google has also published a Tips for Android 2.3 Users page for tips surrounding both the upgrade process and detailing certain changes between 2.3 and 4.0.
Additionally, there are certain apps from 2.3 that will need to be fully upgraded in order to work with Android 4.0.
For users who are looking for a manual update, the file for T-Moble’s Nexus S is live already. Keep in mind that with the manual upgrade you are downloading at your own risk.
While 2.3 is not a bad operating system by any means, the upgrade to ICS is certainly a welcome feature for current Nexus S users.