I like to take a CIA approach to this, and put it on a 'need to know' basis. Some people, in particular, health nuts who think themselves the most righteous people to give dieting advice, are EXTREMELY judgemental. These are also people who have never known hunger like we have, IMHO. They don't need to know. When you start losing weight and people ask you, then tell them.
That's how I'm doing it. It's not like anyone was going to discourage me from doing this surgery, but it helps to not have a bunch of negative ju-ju thrown your way because you told the wrong person.