I agree, right now the screen lock is not really helpful, especially while using in the dark.
In case of activating backlight by touching the screen (which is quite intuitive), I always have the lock-icon in the center of the screen. This is both covering main information and aesthetically bad. it would be far better, as suggested above, if the lock would be smaller and not in the center (maybe transparent).
This would allow using the lock function the whole day with no big issues.
