Dude I had a massive Burn In issue with my iMac Early 2008 (24" 3.06Ghz C2D). There were dark patches all over my screen like dark clouds on that gorgeous screen. I had APP on that, and I turned it in to Apple Genius in July 2009, almost a year after I bought it and they replaced the entire LCD panel and handed me a bill which had a service detail of around $950.00 but my charge was $0.00 because of the APP. Thank God for that!!
This was already talked about on another thread. A recommendation was made to start your screen saver after 3 minutes of non-activity and put the display to sleep after 8 minutes. Additionally, there was an article on Apple's website, the link was on the other thread, where you should switch to a different wallpaper and something about changing the brightness and after a certain period of time the burned in image will disappear. Sorry I do not remember the thread this information was on. Try searching the forum for a similar topic.