I think the most condescending folks are those who work in any trade profession dealing with construction. I guess what they do compares closely with IT in that it's stuff that anyone can technically do (you can buy wire and pipes at Home Depot) but that most people don't have the skills to do. Just read any home improvement site and you'll see what I mean. Sometimes I don't know if they're trying to keep their jobs or just think that the world is filled with retards and don't want to be responsible when someone floods their house. IT is the same way, if all the users were smart and could do things themselves then the help desk would be out of a job.
