Here is a picture of the condensation on iMac 24" I set up a week or so ago. Gizmodo has some more extreme examples of the issue. I am glad it didn't get that bad.
That said, the client is probably going to want to return the unit.
iMac screens experiencing unwanted condensation?
Filed under: Desktops
One of our readers dropped us a note to tell us that his new 24-inch iMac was exhibiting condensation build up behind the screen: he's not the only one, with a few people on the Apple Support forums and elsewhere also reporting the problem. Our tipster says that condensation builds up in the lower corners, goes away, and then comes back when the surrounding environment drops in temperature. Moisture plus computers doesn't compute, so make sure to report the issue to Applecare if you're seeing the same issues. Hopefully Apple will step up and publicly recognize this as a fault sometime soon.
[Thanks, Lee B.]
Read - New iMac condensation inside screen (Apple Support)
Read - Condensation behind iMac glass (DPreview)
Read - iMac Aluminium Condensation (MacInTouch)