[jzwinck]

Just yesterday someone asked me whether a particular disk array would be the right size or not. I didn't have any particular number in mind beforehand, yet I was able to say that the proposal was oversized by a factor of five. If someone told me they had a sweet new application for gas stations in California, I would be ready to figure out how much money each installation would need to make to cover salaries for a programmer and a DBA, even if it's just a hallway conversation. It's OK to be a bit off; it's not good to have no idea.

Your point that interviewers read the wrong signals from candidates' responses is a very good one, but it's not specific to estimation questions. It applies as well to straight-up programming questions and probably a lot more.