I updated to JB 4.1.2 firmware ( I9100XXLSJ ) yesterday and my phone kept complaining about Network Lock. Which i have fixed in few easy steps.
We assume that your phone is rooted ( I may add a tutorial on this later)
1. Download and Install this given apk ( GalaxSimUnlock.apk )
2. Once you open the apk it will show you the current status of your phone.
3. Click the white bar at bottom.
4. If asked grant the root previlages.
5. It will inform you about the network unlock. ( Now simple restart the phone) .
- Mihir Patel
This is the easiest method that i found that works. Let us know if it does not work for you. and post your solution if you have anything better !