I bought a new iMac a little while ago, and I've noticed a couple of odd issues with it. The first is that some shades of certain colours appear to be moving slightly. They have a wavy effect passing over them, and it's quite distracting. The main place I notice this is on my background. Try setting the attached image as your background and look in the bottom right corner to see the effect. The other is that it just seems noisier than my white C2D iMac, the hard disk is quite loud.