The xda-developers forum has gotten its hands on what appears to be the official Ice Cream Sandwich update for the Nexus S 4G. Numbered Android 4.0.4, the ROM's appearance appears ahead of the over-the-air update from Google itself.
While it isn't quite certain if the update is the official one, it's notable that it's slightly newer than the Android 4.0.3 running on the Galaxy Nexus. WiMax drivers in tow, this release is only intended to work with the 4G version of the phone. Google released the GSM update last December.
Even if Nexus S 4G aren't too keen on the idea of doing a risky phone-flashing, the appearance of the update is probably a good indication that the official release is well on its way.