I did a lot of google search for Nokia N70 backlight problem. Many people are facing this problem. Under setting>>Phone>>Display>> you don't have an option to switch it ON of Switch it OFF. Of course it may not be absolutely required.
Another problem with back light is time out delay. I found a thread in Nokia Forum stating the time out is happening after +30 seconds. Meaning, if you set it for 10 seconds the timeout happens after 40 seconds(10+30)
If anybody found a way to switch on/off backllight please inform in the comment section