Blackberry 8310 Backlight

I put the invisible shield on it (from ZAGG, look it up for more info), and I applied it, and I allowed it to sit by itself for 12 to 24 hours like it said in the instructions (I let it sit for 24 hours). However, is it just my Blackberry and I should worry about it or does this happen to you guys too?

(My phone is on default settings). The light of the screen just dims randomly, not to a point where I can't read it anymore. I can still see it, but it's not as strong as it is when the phone turns on and whatnot. Sometimes it goes stronger, but I don't know how to control the backlight strength (in the settings, it's at 100). I don't know what to make of this. Any help, guys?

Nevermind, guys. False alarm. I just checked on my dad's Blackberry, and it happens too. I don't put the Blackberry in its little pouch thing because I also bought one of those snap-on cases/protectors for it.

Just for quick reference: How do I put a screen lock in which you have to input a password to get through (I want it to be automatic).