I have a late 2012 27" imac. I suddenly noticed it started showing image retention. Its a very slightly hint of a blue shapes from high contrast windows thats been visible earlier. It usually disappear after 10-15 mins, but its nonetheless annoying. Some tiimes its not there. I have ordered a Retina iMac, so its mostly because of re-sell value I worry about this. I have no clue if it has been there all the time, I would assume I would have noticed. Does anyone have any experience with this? Where is the limit before Apple replaces the screen? I have Applecare which still has 1+ year of warranty. But they would have to do some serious testing for it to appear, because it doesn't always show up.