I've never understood the whole "starvation mode" thing; I'm sure that it can happen, but I think the term gets thrown around way too much. As this site often says, weight loss is all about calories in being less than calories out, so I'm sure these stick-thin "celebrities" just don't eat. And do drugs, which would help with keeping off pounds.
I actually heard that Nicole Richie had a scale at the door of her Memorial Day party and no girls over 100 pounds could come in. Sounds like the kind of people you'd want to be friends with, right?