When I saw 'up' today there was a short before hand. It was about Cloudmen and Storks. I can thin of a few guesses as to why it wouldn't be there.
When I saw it it was in Disney 3-D. Is this the way you viewed it?
How early in the day did you see it? Sometimes the people doing the reels of the movies make mistakes and forget that there are reels before the movie but after the previews. Possible that they forgot it. For instance I never saw the Bird short.
Lastly some theatres are just crummy and don't show the short so that they can squeeze in one more showing of the movie.
I hope you can see the short for this movie soon as it was adorable.