WDW is very accommodating for dietary needs. As mentioned, for TS make sure it is listed on your ADR, and then check again with the hostess stand when you arrive at the restaurant. If the restaurant has one of the new(ish) allergy menus, then they should give you that which covers most major allergies, gluten, dairy, nuts, etc. If they don't have one of those menus, or you have a less common allergy the chef should come out and go over your options or walk you through the buffet. At a CS tell the cashier, and they will usually have you speak with a manager, or bring out "the book" (contains ingredients for everything in each dish). Your wait may be longer depending on what is involved with making sure your food is safe. To get an idea of what WDW can offer you, check out this blog -> http://www.glutenfreedairyfreewdw.com/
Thanks so much! It's wonderful to know how accommodating they can be! I always feared to be one of those people who had to alter their choices because of an allergy, but I'm so glad to hear of the options I have now.