Users have to wait a while longer to Google release the next version of Android 4.0 (Ice Cream Sandwich) for Nexus. But recently, kiwiboo who is a member of XDA forum released a build for both Nexus S and Nexus S 4G. This Rom is AOSP (Android Open Source Project). There are still some small errors such as user must manually configure the wireless network, camcorder don't work, can't connect USB, but users have had positive feedback. You can download ROM here to try ICS on Nexus S.