Since 2008, I've been suffering iMac display issues like image persistence, grey stains, etc. My history includes five replacements of LCD panel, one replacement of HDD and one replacement of the whole iMac. Recently, I added another replacement of the whole iMac(early 2008) to a new iMac(mid-2011).
Mostly, my complaints were about image persistence, not smudge or dust, and I believe the image persistence is due to poor heating control of iMac systems at the time. Regarding new iMacs, however, it seems to focus mainly on smudge, dust, or as some people called, cloud. So, I visited an Apple store to check the new iMac. Here, I could find the grey stains even in iMacs displayed the store. The clerk looked slightly stumped and indicated the upper side of the iSight camera. To prevent dust to flow inside panel, he told me to have to tape the position as soon as I unpack the new iMac. It was on the built-in microphone to be taped! I've never noticed the mic. until then. The mic. seems to be located off the thickness of the tempered glass screen.
Then, it means that the Apple has already recognized the design flaw, but they never admit it officially. If it is true that the mic. was the cause of the grey smudge, Apple must admit the design flaw and recall iMacs ASAP!