this problem seems to come from the inside of the screen not the outside. apparently you need to take it to apple to fix. this bothers me since the imac is the size of a uhaul truck and the nearest applestore is in a mall. so i dont want to be walking through the mall with this huge thing to have to get it fixed.
I asked this a similar question about two months ago before buying my first iMac. You can find the thread here: https://forums.macrumors.com/threads/1303359/
Long-story short, it doesn't appear to happen to most folks, but it does happen and get talked about frequently. The main culprits seem to be a combination of heat, dust, and humidity which cause the slow development of black smudges in the inside, upper right-hand corner of the iMac screen -- basically in front of the graphics card. Most people believe that the next upgrade won't fix this at all, that it'll take a major redesign as the iMac casing & design is just too "tight" in that area.
FWIW, I went ahead and bought one anyhow, and I love it. I've had it for two months now with no black marks to report, though from what I understand, they take time to develop.
If you do buy one, be sure to buy AppleCare.
thanks crouch. I wonder if that mark that develops if its by the video card an actual burn mark. if you hold up extreme heat to glass it will blacken. hrm. I will have to think. I havent decided yet what my next mac will be.
Alternatively consider the Mac Mini + Thunderbolt display. The fact the Thunderbolt display does not have any hot CPU/GPU components inside along with fans pushing hot, dusty air around inside should mean the display doesn't develop this issue. Another plus side is you can change/upgrade the computer at a later date and keep your display, making the upgrade cheaper.
I thought about this too -- buying the Mini + an ACD instead of the iMac -- but I started doing research and learned that ACDs have the same problem:
https://forums.macrumors.com/threads/1183925/
https://forums.macrumors.com/threads/1079993/
https://forums.macrumors.com/threads/861355/
It's frustrating. What's worse is that Apple seems to blame customer's environments for the problem.
How widespread is the screen dirt slash blotches problem. it seems like there is alot of posts to worry about. would this be considered a defect somehow and something to avoid until the next mac update.