I've had a 24" Silver iMac since Aug '07, and just yesterday started getting burn-in issues. I think the room temp got too hot, and also the iMac. And then since my son left Photo Booth running on his account, I think my screen saver wouldn't cut in. I came back after a few hours last night to see screen on with content, not the usual Flurry saver or black screen.
I've never been obsessed with screen saver, under the false assumption that LCD's don't have burn-in concerns. Just got a quick education on that error, and on the relatively low quality control in Apple production. When you make good looking stuff, I guess you don't have to make it work well underneath in every regard. Style over substance... in Apple's case: form and function over fortitude.
So, now I've learned of JScreenFix, smcFanControl, etc. etc. I have a white screen as saver right now, and we'll see how that goes. I'm gonna blow out the collected dust in a minute, but I'm afraid to get suction cups, pull out the glass, and massage the LCD. Are there any other material remedies that I've missed?
Will report back any beneficial outcomes. Just wanted to chime in and increase the occurrence counter on another iMac with screen burn-in. BTW, mine's an early Silver edition and the screen's great otherwise.