According to the "Everything Mac" booklet:
1) switch off your iMac and unplug all cables
2) clean the screen with a soft, clean, dust-free cloth (little moist)
do not spray any liquid directly onto the screen
I saw your new message after I posted my first reply.
After I got my new iMac, there appeared some fog (or something) behind the screen. It dissapeared again after ~ 2 hours of use. It propably was some moisture between the glass and the screen itself.
Any chance you're having the same issue?