its not with the ambient light, i have that feature turned off. i mean sometimes, i hit the buttons, it doesnt come on. unti l30 seconds AFTER ive hit the buttons. then it decides to pop on.
That is completely normal. There is a light sensor where the blinking led(coverage indicator/missed message alert) is. Try this, go into a dark room, the keys should light up. You can also test this by covering the blinking led with your finger to trick the phone into thinking it is in a low light area.
If there is light in the room your keys wont light up just because you press a key or use the trackball.
Mine does it at anytime. I can be sitting in one spot, the ambient light doesn't change and my screen will darken a bit and then about 30 seconds or so later it brightens back up.