I heard "1 in 5" which seems like quite a bit. Now, this IS a forum so generally more people post about when their computer goes wrong rather than when it works fine - but there do seem to be a few issues with the new computers. I've heard of people getting great first computers, and people who had to go through 3-5 computers before they got a good one. Make what you will of that!
I don't hear much of discoloration or chipping anymore, it's mostly mooing, random shut downs, bad logic boards, etc.