Basically, Apple doesn't have a clue what the problem is. All pointers suggest it is hardware and personally, I wouldn't rule out the main board just yet.
Some iMacs being received this week are freezing; I'll let you make your own minds up about whether Apple is knowingly shipping faulty hardware.
I'd also suggest that speculation about the scale of the issue is spurious; just as those without an issue stay away, those who are suffering don't all post on MacRumors, Apple (they make their forums difficult to navigate to for a reason...) or any of the other main Mac discussion boards - they may not know where to go, their issue is so bad they cannot even go without a freeze long enough to post, they don't have internet access and they may even have an aversion to using a discussion board (as many still do).
Talk of 5% is silly; it is obvious that a huge number of new iMacs are faulty. We know it is in the thousands based on the number of posts across the net, possibly into tens of thousands. And that is a number just based on those who know where to complain. Just because you or I post here, doesn't mean to say everybody else does!
Finally, talking in terms of odds of getting a working iMac? Seriously, when did Mac users start accepting bookmaker set odds that we might get a winning machine? This is awful, we should be demanding the best; Macs aren't cheap and we have all sat idly by while Apple's QA has gone down the pan in the past 12 months. Hardware faults and software bugs are increasing all the time.
If we just accept mediocrity through a love of Apple, we only have ourselves to blame when we end up in the same boat as Dell buyers. That is to say we just expect problems, whether they be our Macs suffering through faulty hardware, our O/S having no firewall, iPhoto just doesn't work or our iTunes rejecting our iPods.
I came to Apple all those years ago to avoid that scenario. I don't expect to start suffering that as a consequence of the company becoming too big for their boots.
And neither should anyone.
