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.
I am on my macbook right now. It is wonderful. A little hot on the bottom but not more than my brothers hp laptop. I talked to a lot of people that had macbooks and they all had good ones. The majority of all macbooks are amazing computers, but the bad news sticks out more than the good news. Also when someone says something like "macbooks are loud" people start listening really hard for sounds. The fans are barely noticable if you have any kind of noise in the background, I have never heard a macbook moo, and I have used eight in three different stores. When people say they are hot people start freaking out. most of the problems are people blowing things out of perportion worried that something bad may happen when there is really nothing wrong. I say get a macbook, I love mine.