I have to admit the thought of installing this update still makes me nervous, but I do genuinely appreciate the clear and detailed explanation of what happened. Contrary to what others may suggest, as an administrator I can appreciate the complexity of testing code before its release, and there are always things that get missed. That's why continued improvements are made. However, I have generally followed the idea that update rollups should be a collection of bug fixes and should NEVER introduce "new features"... save new features for the service packs.
I would also point out that while customers are right to be upset about this type of mistake, they must also take responsibility for their own implementations of software into their environments. This is why we are encouraged to run test lab environments and thoroughly test any significant update before putting it into production precisely because every environment is different and generate different scenarios than those Microsoft is testing. Microsoft makes it very clear in the KB articles what has changed and why, so that makes testing an update easier for us... we just need to actually do that testing.