I pre-mix my daily shakes, soups, whatever and bring them to the office. Since I have the Magic Bullet which comes with two lids, that allows me two shakes at work, plus a soup and a bar. My 5th meal of the day I have on the way to work - a shake without the lid. And then dinner.
Most people don't pay too much attention to what I'm doing, but I'm with you - you don't want the whole world knowing what I'm up to. Makes it feel like you are pressured to show results - I'd rather let them notice LATER that I'm losing weight. I work with a lot of women who drink Slim Fast or eat lean cuisines, etc. so it isn't exactly a hostile environment. There are certainly people I wouldn't tell though - the insensitive ones - you know who they are ;-) Think there is at least one at every workplace - even if they are well-intentioned, they can be rude or hurt your feelings!