I've had this problem on a mid-2011 iMac, I believe it was.. I had to take it in to Apple to fix it a few times. They ended up giving me a new iMac after the first time, because whoever worked on my computer scratched it up.
The second time I took it in, the guy told me I should keep an air purifier near the iMac, which I did/do.
The third time I took it in, the guy tried to tell me it was my fault and that water must have been spilled on the iMac or it was in a very humid room. They ended up fixing it after I told him he was full of ****.
So, what I gathered from the folks at my local Apple store was that, as Mossy said, the dust gets inside the panel from that vent in the top/back of the iMac and it seeps through the panel since it's just magnetized.
SOoo.. I got a new iMac.. which doesn't have that vent at the top and is secured with an adhesive.. and now it's on all 4 corners, plus two more areas at the bottom, of the screen, about one inch to the left and one inch to the right of the Apple logo.
I will be taking it in, but I am going to wait until after the holidays.