I understand that it is not possible to re-lock / lock / relock the bootloader on the XT907 due to the method required to unlock it. [If this has changed, please let me know!]
But is it possible to disable the Motorola message at boot about the bootloader being unlocked?
Some material in other forums made me think that it might be an image that can be flashed over with the original image so it looks stock again. This would be a solution for hardware warranty matters unrelated to rooting after unlocking the bootloader.