At least with my G1, yes - once it was sim-unlocked, nothing I did to the phone changed that. I think the firmware level at which the sim locking occurs is unaffected by the roms you have on the phone, though others with more experience may correct me on that.
It should stick. With other manufacturers I've used, and I don't expect samsung to be any different, it seems to go beyond even firmware. It will definitely survive OS updates and history has shown it survives radio updates as well.
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