I just installed the ICS ROM ported (with permission) from Koush's official Nexus S ICS build.
You can get it here:
[PORT][WIP ROM][NS4G] Koush's ICS Build for CDMA [Koush Alpha 10 Based] - xda-developers
Note: you'll want to install the gapps ROM, too, in order to get full functionality.
Just curious if anyone else is using an ICS build on NS4G and what your experience with it is.