They had it in a green plastic coffee can, with a twig and some grass and leaves. It was clutching some blades of grass by what I assumed was its mouth. It turned out I was wrong. The grass was being clutched by the caterpillar's butt end.
Note the strand of silk in the image, cutting across the "o" in "(S)wallowtail". There was also some silk woven onto the inside of the coffee can. Apparently, the caterpillar had decided to form its cocoon right then and there, inside its little green cylindrical habitat.
The caterpillar appeared quite agitated, crawling all over my hand, my shirt, and the strap to my camera case. My cousin grabbed the camera and started taking pictures. Here's one of me going eye-to-eye with the caterpillar:
UPDATE, 7/25/2007: When I checked this morning the can was knocked over and the rubber band holding the net over the opening had been removed. But there is a cocoon in the can now! The caterpillar used one of the leaves to wrap the cocoon and it would be hard to see if I weren't specifically looking for it.