I would say I usually lose a little, I'm not a big eater anyways at the best of times, although I do like Disney food. But with all the walking and swimming I'd do at Disney that I wouldn't necessarily at home (although I am quite a gym bunny and a cheerleader!) I think I end up burning off more than I consume most of the time.
However on our last trip to WDW in August 08 I had just started coming down with Crohn's which has made me lose an awful lot of weight, to the point where I am now malnourished and on a special liquid feed. So I don't know whether that may have factored in at all.