Google might be already planning to release Android Ice Cream Sandwich update for Nexus S phone. But kwiboo, a member of XDA Developers already came up with a build which can be installed on Nexus S. Days after announcing Ice Cream Sandwich Google released source code of their latest mobile OS. This opened up lot of opportunities for developers to modify source code and load onto any device.
As this is based on Android Open Source Project you may not expect all the feature set works as expected. But it seems to be pretty positive update for most of the people who already downloaded and tried. Few minor known errors like USB and data usage statistics most of the major functionality seems to be working as per the developer who released the build.
You can download the builds from here. Make sure you read other users comments in the thread on XDA site before you install the update. If anyone having issue which is critical for you then you can avoid installing it.
Most of other latest Android phones are due to get Ice Cream Sandwich update by early 2012. Samsung just announced to make ICS available for Galaxy S2 soon.