iFile from cydia (free trial - $4 if you want to keep it) allows you to view this plist.
I highly advise against editing the plist with anything else than the command line utility I mentioned earlier.
I tested with my own 3GS and ejected the sim. The carrier field stayed present. It should update with the correct value after you put in the new sim, or after a restart of the phone.
Thanks again for your informative advise Olethros. You were right all along. I went to a Movistar shop yesterday and one of the guys working there was nice enough to let me use his sim card. Right after a few seconds searching for a signal, my iPhone was able to connect to Movistar. I checked on the carrier's name under settings and it changed from "Softbank mobil 5.0" to "movistar 5.0" Don't really know why the 5.0 appears but it seems the unlocking worked! :)
So there you go. For anyone out there with a Japanese iPhone 3GS, ultrasn0w will make the trick and unlock your phone without the need of Softbank sim card.
Hi there! I have jailbroken my two iPhone 4 with iOS 5.1.1 and iOS 5.1 (both locked in to Japan-Softbank carrier), and I have tried to do the repo666.ultrasn0w unlocking method via Cydia. But it is not recognizing my Philippines sim/carrier. Is there a problem unlocking the Softbank (Japan) carrier? Or is there a problem with their iOS? Hope to get a response. Thank you!