Well, since the device will have to have an airplane mode there's obviously a way to have the GSM radio off. The only trick is convincing it to turn the GSM radio off and leave WiFi / Bluetooth alone. Now if you're really gonna be paranoid, you'll want to change yout WiFi and BT MAC addresses too...

I agree with everyone who's posted they assume that if there's no SIM it will be in an offline/flight mode and who assume that this means the radio's off and it's IMEI can't be accessed by a GSM network. However, I wonder if anyone knows for sure? I am new to this AMemo lark, so I don't know if this sort of thing would be handled by the open-source bits of the OS and, even if I knew that, wouldn't be able to understand the code...

gsm driver is closed source, afaik, thus you can't really know what's it doing (unless you dig deep ).. on the other hand, device in 'aero mode' is probably required (by laws etc.) to remain silent if engaged, so...