I had the exact same problem last night. Decided to go back to stock after flashing ICS last week, then going back to B888 via the IMEI hack. Managed to reinstall B882, then OTA to B885, then a second OTA to B892. Wi-Fi was fine up until the final OTA, the phone flatly refused to connect to my router. Luckily, when i first got the phone i'd made a CWM backup after the initial OTA to B892, so i installed ClockWork Mod and flashed the nandroid backup, Wi-Fi working fine now. As it was mentioned further up this post, it seend there's a bad driver issue, or, as was also mentioned, ICS leaves a footprint on the phone which means a true return to stock isn't possible. I'm leaving well alone for now.