Had what I can only call a sinking feeling earlier, luckily it was not as bad as I thought. Noticed a mark on the outside of the screen, so grabbed damp cloth and gave it a wipe. After a few moments it was pretty clear that all was not well as it looked like I had scratched the screen, so I got some screen cleaner and another microfibre cloth and no matter what I did nothing would shift what looked like a scratch. In fact things got worse, so bad that I thought the screen was ruined by the cleaner as there were marks everywhere I could not shift. In desperation, I disconnected all the cables and laid the iMac flat, back down on my bed and grabbed at the screen edge and managed to overcome the magnets and remove the screen. I then moved to another room, to avoid dust, and went over both sides of the screen. The marks disappeared, put the screen back on and all is good again. The marks were apparently on the inside, I think caused by static buildup on the front causing dust that had go into my iMac to become attracted to it and stick to the inside of the screen. My iMac is 14 months old, and used in a fairly dust free environment. Now to the question, what does everyone use to clean their screens. After todays happenings, I don't want to scare myself like that again.