As with everyone else, I had a 27inch, late 2009 model. I had it for roughly 8 months and noticed it had the grey smudges in the top right corner. It looked like a 'ghost image' i get sometimes on an older dell lcd screen. However it never went away. Since I was having an issue with my hdd fan speeds, I took it in to my local apple store. The guy tried to clean the screen, no luck. He said the screen would need to be replaced, and since it was still under warranty I wouldn't have to pay the $800 for the part! They called me the following day stating everything was done. When I went to pick it up, they didn't replace the hdd like I had request (had a kb number they ignored). So they kept the imac for another day. When I went back the following day, they had damaged/scratched the casing in the front. After a 'bit' of talking, I walked out with a new 2010 imac, So far there are no issues. But from what I have read in this post, I don't have high hopes that the marks wont appear on this model.
If they charge $800 just for the part (that doesn't include the labor), they should be absolutely sure that there are no issues, or at least do some sort of volunteer recall to fix/solve the problem, instead of selling products with apparent issues.