In my recent experience regarding this issue, I think that Apple must consider this problem seriously.
Last June in 2008, I purchased the 24" 3.06GHz iMac and one month later, I found the black lines(persistence, burn-in, or whatever called) at the upper-left position of the display. I guess the black persistence came from firefox toolbar line.
http://www.flickr.com/photos/sontop/3220884549/
The service center evaluated as the LCD panel defect and replaced it with new panel on November, 2008. It seemed okay but just one month later, the exact same black persistence at the upper-left position occurred again.
http://www.flickr.com/photos/sontop/3220891379/
Since I strongly insisted that my iMac should be replaced entirely, now I have new iMac with new 1 year warranty. But I feel still skeptical whenever I look at the upper-left part in the display, which makes me irritated. Also when I read many postings about this iMac display issue in various Apple-related forums, I have strong misgivings that I will meet the same issue again near future.
It seems to happen mostly in 24" display with NVIDIA GeForce 8800 GS card. Also at the upper-left position(bad arrangement or heating). Now I do everything you guys suggest-white screensaver, fan control, turning down LCD brightness, etc.
BUT above everything else, the Apple should find out the problematic part-lcd panel or graphics card or whatever-ASAP, then those must be recalled!