To get into the program I did have to write a statement of purpose that declared what area I want my thesis to be in, however once you are admitted they advice you to throw those notions out of the window and to use the two years before taking qualifying exams to explore a number of different areas. I was basically told that, just as in undergrad, most people change their minds. Sometimes this is because of the professor who becomes your advisor and sometimes because another class sparks your interest.
I wrote the statement of purpose on business reglation and the impact of government policy on the markets. I think I want to stay somewhere in the business side of public policy since my previous experience is working in the markets, but I am taking my advisors advice to heart and actually none of my classes next semester are business related.
Ask Me Anything is still open. So if you have any other burning questions for me, now if your chance!