If you've downloaded or had the Android 2.3.6 update pushed to your AT&T, T-Mobile, or unlocked Nexus S recently without issue, consider yourself lucky. None of this applies to the Nexus S 4G on the Sprint network.
While we reported that the update was breaking Wi-Fi and USB tethering initially, it seems something much, much worse is happening to some users who have received 2.3.6 OTA.
If you check out this Google thread, you'll see a number of poor souls have had all cellular connectivity stripped from their devices after updating. The easiest solution so far? Roll back to Android 2.3.4 (or earlier), which almost inevitably requires a full wipe of the device.