Almost two months ago Apple announced a series of updates to the iMac line (see "," 20 October 2009). Changes included larger screens (21.5-inch and 27-inch displays), SD card slots, and overall upgraded specs - most surprising of which were the first quad-core processors to be made available in a Mac outside of the Mac Pro and Xserve. At the time of the announcement, Apple noted that the i5 and i7 quad-core models would be shipping later than the rest, in November 2009.
As eagerly awaiting customers have begun to receive their shipments, a disconcerting trend of cracked screens and problematic displays has emerged. A on the Apple Support Discussion forums discussing the problems has received over 32,500 views and nearly 200 responses - significant numbers that indicate widespread affliction. Yet, user forums are often difficult places to synthesize information regarding problems like this, as specifics are generally muddled within anecdotal paragraphs. Thankfully,  (hosted on the anonymously run imac.squeaked.com) has brought clarity and analysis to the cases presented in the Apple discussion forums.
The iMac Issues site disclaims that "The data presented here is based on information submitted by people on this Web site or taken from comments posted in Apple's Discussion boards (this is an 'unscientific' survey and as such should not be considered representative of all iMacs sold)." Be that as it may, it does help clarify the available data.
Looking at the Numbers -- While a cracked screen is a more startling problem and makes for a more arresting story (and, fortunately, an easy replacement), most users complaining about their displays are in fact affected by poor video output. Of the 482 cases logged by the iMac Issues site, 71 were related to cracked screens while 225 revolved around flickering displays. Other symptoms listed include a yellow-tinted screen (often appearing as a band on the bottom portion of the screen), dead pixels, and an inability to boot. Descriptions of display issues also include the appearance of the display tearing or splitting, or of its image becoming offset and distorted. 78 users are counted as having absolutely no problems.
Affected systems are far more likely to be one of the 27-inch models, especially when concerning instances of broken glass. Of the 405 cases identifying some kind of problem, 374 involved a 27-inch iMac, while only 31 involved a 21.5-inch iMac. When the problem identified is broken glass, all of the cases involve a 27-inch iMac. While it makes some sense that the larger model's glass is disproportionately affected because its extra screen space makes it that much more vulnerable during shipping, it's harder to explain the discrepancy between the video output issues, unless the numbers simply reflect the 27-inch model's popularity relative to the 21.5-inch model.
Examining Causes -- No definitive conclusions have been reached regarding the causes of these problems. Especially mysterious is that in the majority of cases with broken glass, the packaging appeared unscathed. We assume the computers are going into their boxes in good condition, which leaves either excessive fragility or issues with transport and packaging to blame. With the packaging itself usually showing no signs of distress, figuring out exactly what's happening inside the boxes en route becomes a puzzle.
As for the flickering and problematic displays (), suggested causes, or at least avenues for further investigation, include high computer temperatures, defective power supplies, incompatibility with AirPort base stations, and an issue related to brightness settings - dimming the monitor appears to reduce the symptoms. Yet for all the speculation, consensus on the issue has yet to be reached.
What You Can Do -- If you are affected by the screen flickering issues, be sure to contact Apple, either  or by working with an Apple Genius at a retail store, to ensure your problem is logged. Also consider adding your experiences to the ongoing Apple discussion forum thread linked earlier or  to the iMac Issues site's survey.
For those with broken screens, Apple has been replacing these iMacs without hesitation - though a few unfortunate customers have received replacements that were themselves broken. If you have an Apple retail store nearby, consider seeking your replacement there until this issue is resolved.
We hope to see Apple address what are clearly serious and widespread problems quickly and fully.