[XPERIA X8 Q&A, Help & Troubleshooting] B.Jay:
My X8 came SIM unlocked by default. Since it's old enough, and therefore safe for boot loader unlocking, I gave it a try (even been as daring as to unlock while having GDX v021 flashed).

Connected and checked with s1tool that it's suited for unlocking: Pass

Unlocked the boot loader: Pass

Booted up (SIM does not have PIN1 set)...

Connecting to the GSM Network: Pass

Connecting to the APN for 3G Internet access: Pass

Reboot (to make sure it didn't brick): Pass

Re-locked boot loader: Pass

Checked with s1tool that boot loader is locked: Pass

Device still able to connect to GSM/3G: Pass

I didn't try to flash a custom kernel but just stayed with the stock kernel. During the whole testing scenario - from unlocking to re-locking - I didn't have any problems with GSM/3G network connectivity or troubles reading the SIM.

Like I wrote above the PIN1 check was disabled because sometimes GingerDX shows the "Network unlock" screen after inputting the PIN ... that's more-or-less a bug workaround.

Thank you for correcting the spelling of "Omnius" ... I was typing it from the top of my head and failed to recall how it's spelled.

If you refer to "how is the SIM lock lifted" then I think the best persons to answer this question are the guys who cracked the boot loader. I don't know enough about how the low-level init of the phone really works ... the boot loader guys aren't very chatty about telling us how the low-level boot of the X8 works.

Since, in my case, the SIM unlock prevails when flashing a custom ROM on top of the stock kernel (keep in mind I never flashed a custom kernel, so I never altered the "baseband" partition) the unlock can't reside in the ROM partitions.

But ... as the problem only arises when tinkering around with the boot loader but didn't affect my phone during boot loader unlock / re-lock ... and this is now purely guessing ...

The boot loader unlock only replaces the boot loader binary with a altered one. So if the thing that tells the phone that it's SIM unlocked resides in the part of the NAND where the boot loader / kernel / baseband is located it can only be a digital certificate or some sort of a binary that gets loaded through "Stage 1" init (during which the boot loader and kernel gets loaded and all the security checking stuff happens). It can't be in the higher init routines where the initialized kernel is boot strapping the ROM as that would disable the SIM unlock when flashing a custom ROM.

If blagus or the kernel devs have some ideas it would be great to read a few lines from a source knowing about the low-level init routines (to actually understand how the X8 comes to life once the power button is pressed).

On another thought ... the msm7227 CPU also must have some kind of firmware (think of it similar to a BIOS like a PC has) to init itself and the hardware ... maybe the SIM unlock is somewhere in there?

EDIT: On a third thought ... it would seem almost logical that the SIM unlock stuff is stored somewhere in the "security" related part of the NAND (somewhere deep down at the low-level init). If the phone came locked and got unlocked at some point the original SIM lock will most likely spring back into place at boot loader unlocking - so the SIM locked/unlocked thingy has to be somewhere in the low-level boot init; otherwise it wouldn't disable a "soft unlock" once the boot loader got unlocked and replaced past the cracked one which bypasses a whole lot of the security checks so unsigned boot binaries (kernel) can be loaded and executed.