What build of ICS do I need for Nexus S baseband I9020XXKF1
My baseband is ever so slightly different from yours: it is I9020XXKI1 (I think; it's hard to read).  I upgraded by following the steps at http://www.androidcentral.com/how-manually-update-your-gsm-nexus-s-ice-cream-sandwich; I am pretty sure that you can't go wrong doing the same.  At worst, your phone will refuse to install the thing.