ember - 3:30 pm on Jul 4, 2011 (gmt 0)
We just recently had one ad unit on one page on one site disabled. I worried that it was the beginning of something bad, but it was a month ago and nothing else has changed. After some checking, I found that the traffic to that page was probably not to Google's liking, and so they disabled the ad. We removed the ad unit and have continued on. So apparently they can disable individual ad units if they want to.