Up art director Lou Romano has posted some of the early development and production work he did for the film on his blog, and it's fascinating both for its own beauty and for the glimpse it gives into the gradual evolution of the movie's look.
Meanwhile, the folks over at slashfilm have completed the arduous task of mining Up for the Easter eggs that Pixar delights in scattering throughout its pictures. So if you missed the Pizza Planet truck or the glimpse of an (apparent) new star of Toy Story 3, you can find them here.
--Christopher Orr