Customers have piled into the Apple Support forum to complain about their busted machines, but Apple has yet to determine the root of the problem. It would seem most likely that the cracked displays are a result of insufficient packaging, poor handling -- or both. The DOA machines could also stem from improper shipping practices, though it is less probable.
Thankfully, Apple is replacing all cracked or otherwise defective iMacs free of charge. As noted by Engadget, if you plan to give someone an iMac for the holiday, you may want to test the system prior to gifting.