While looking for chilled soup ideas I came across this article in the New York Times called Chilled Soup That Really Tingles. It outlines the principles of making really good chilled soups, and some of the common mistakes people make. It also has a couple of recipes.

One thing to keep in mind is that cold soups need more seasoning. They tend to require quite a bit more salt than you would normally use for the amount of soup. Tasting and adjusting your, adding a little more at a time and tasting again, after the soup is cold and ready to serve, is key to getting the flavor just right.