Right now they'll treat the foods they are tired of as if they are much further away, but they won't walk across the fortress to get a new food if an old one is really close by in comparison.
edit: To clarify -- for foods they like, distance is divided by 2 and then 30 is subtracted. For foods they are tired of, distance is multiplied by 4 and 100 is added. So they will walk quite a way for novel foods, but if they are standing on top of an old food and the nearest new food is more than 100 away, they'll go for the old one, unless they like the new food, in which case they'll go up to 260. Of course, all distances are just 3D manhattan distances, since it can't afford to do a corridor-based distance calc here.
edit 2: Also, cooked foods are counted provided they are all made out of one ingredient. If a food is made out of two or more different item types, the system stores it as "generic prepared food" which adds a buffer against boredom for a few years. So you can't escape by using plump helmet roasts, but if you have two food items you can escape by preparing them. In general, three types of food (without cooking) is enough -- dwarves will get sick of a food if they have to eat it five times within a few years provided they have only eaten two types of food. They don't mind eating something five times provided they have eaten two other types of food or a two+ different item prepared meal within the last few years.