I just bought a PB 12 model and after I started it up, I noticed the screen had a gradation from dark at the top to light on the bottom. Being a desktop user, I thought I just needed to get used to the viewing angle of the screen, but in any position, it still has this effect. I called Apple and had the screen replaced but it still looks the same to me. Do PB screens exhibit this tendency or not? Granted, I am used to larger displays but it doesn't seem right. Anybody experience this? thanks.