When I got measured for my wedding dress last month, I asked what the official way of measuring your waist is (so I can do it myself in future for catalogues etc) and they said that the waist measurement is always taken at an inch above the belly button by dress-makers and clothes manufacturers.
Incidentally, I said to them I had always thought that my waist was just the slimmest part and they said it isn't.
Apparently a lot of people think the waist measurement is much higher up than it is too, to the point where they've had a lady before phoning in her under-bust measurement as she was slimmer there than at her real waist, so she thought THAT was her waist measurement! x