iMacs from 2009 to 2011 were released with flawed LCD panels that, over time, developed dark stains and smudges that can't be removed. Apple has failed to own up to this problem in a consistent matter, and while somebody had the luck to be assisted by the Genius Bar satisfactorily, many customers were left out in the cold.

Apple should provide free replacements for those faulty panels even for out-of-warranty machines given that it's an obvious manufacturing issue.

See https://discussions.apple.com/thread/2300580?tstart=0 and many other threads and complaints all over the web by doing a simple google search like:
https://www.google.com/search?aq=f&gcx=c&sourceid=chrome&ie=UTF-8&q=imac+screen+stains

There is another online petition, please visit that one too at http://www.dirty-screen.com/en