Up, the upcoming Pixar release, is still a bit unexplained and in the dark. No one is quite sure of the characters, plot, or setting. One thing that is known about it, however, can be found in Ratatouille. While listening to the commentary of Ratatouille, about 15 minutes into the film, Brad Bird refers to the shadow of a dog on a wall. He remarks that this dog will be in "the next film after WALL-E." This film, of course, is Up. This shadow is pictured above.
Source: The Pixar Blog