My dad just bought a 24" iMac for use with his design company as his main machine. While installing his new software, I noticed this band on the display. It was visible at startup, but more or less disappeared after. I started to notice it as I was applying a firmware update.
Has anyone seen this before and was able to find a solution? I would really, really like to avoid taking it in for service, so I'm hoping it's a software problem that can be resolved easily.
Set the background to a solid colour, and see if it still happens. The problem with screen discolorations/banding is that they can be almost indistinguishable on a screen full of content / different colours, but they will stand out like a sore thumb on solid colours (e.g. the startup white background)
I also recommend taking it to be serviced, since that seems incredibly unlikely to be a software issue if it's occurring during startup (i.e. prior to display drivers being present, since EFI uses very simplistic techniques to display content during startup - it doesn't depend on hardware drivers etc) unless you're certain it wasn't there prior to the firmware update (but that must be one weird firmware screw up if it caused that)
