I recently took the Optimus S to Europe and had to make it an "offline tablet that could still do everything via wifi".
I never had error 67, running HydroROM.
In Wireless and Networks, I went to "Mobile Network Settings" (make sure phone isn't on flight mode or that option will not be there)
Uncheck "Data Enabled", "Data Roaming", and you can check "Data Roam Guard" but that doesn't matter since there will be no data enabled.
THEN, you can switch the phone to flight mode.
Now, enable Wifi (you can turn on Wifi and Bluetooth while in "Flight mode").
I have Google Voice numbers, so I bought GrooveIP. This was the best thing ever in Europe. So long as the wifi hotspot allowed port 5222, GrooveIP signed into my Google Voice account and let me use the phone to call anywhere. Best part? Calls to the USA were 100% free, since as far as Google was concerned, I was on wifi and calling from a US GV number. We kept in touch with everyone in the States for free.
For texting, you can use the Google Voice app linked to your GV number. You text via wifi via that app and can receive texts so long as you're on wifi. So we'd text people from one hotspot, tour around, get to another hotspot and get all our messages.
I didn't test incoming calls, though. I think there is a setting in Google Voice online (
http://voice.google.com) that you may have to change regarding forwarding to your phone on calls or something.
But with GV and GrooveIP apps, the Optimus S did a damn fine job as a little offline tablet. Coupled with offline maps from Google Maps, and it was just perfect for wandering the streets of Europe. We just had to make sure to star every location we wanted to go to in Google Maps when the "OptiTab" was online, so we'd at least know what direction to walk in.