I find some of the responses in this thread suprising. If my seat mate wants to chat a little I do not mind at all. I am a primary care nurse practioner for a major HMO in the San Francisco bay area. If I was not a "people person", I would be in the wrong occupation.
Being social and friendly has almost always worked for me. I do not mind talking about my line of work, it often leads to interesting discussions about my seatmate, or some point about the state of health care in our country. I respect body language, I would not interupt someone working on a spreadsheet. Many people on board an aircraft are flying for different reasons. Since I travel almost always for pleasure, I enjoy meeting, and interacting with new people.
At work I routinely ask people what they do as part of a new patient history. Even in the privacy of a medical exam room there are people who do not want to discuss or disclose their employer. I wonder if some employer's warn there employee's not to discuss there work casualy. I was thinking of those firms with big government contracts, invested in Iraq, for example.