After a long wait, finally Sony has released an official ICS update for AT&T Xperia Ion (LT28at), a week after the international version (LT28i/LT28h) get the update. The OTA update come with Android 4.0.4 (Ice Cream Sandwich) with build number 6.1.C.1.105.
When the phone is connected to the internet you’ll receive a notification about the firmware update or when you connect the smartphone via SEUS. If you don’t get the update notification, you should check it manually in the Settings menu or trying to connect the Xperia Ion with SEUS.
If you still do not get the notification, the other solution is to re-flash your phone to stock Android 2.3.7 Gingerbread firmware with Flashtool and then perform the OTA update or by downloading the official Android 4.0.4 firmware and then flash the firmware to your phone using Flashtool, but make sure you’re on Gingerbread before flashing. Don’t know how doing this, follow the guide here.
Update: after releasing an update with build number 6.1.C.1.105, today Sony will release a minor update with build number 6.1.C.1.107 scheduled to roll out on September 20, 2012 at 9:30 EST.