It looks like he (or she) is the very special Hyalophora cecropia - the precursor to the Cecropia Moth, aka Giant Silk Moth. Reported to be North America's largest moth, the female moth can reach a 6" wing span. The caterpillar is rarely seen during the day as it is mostly nocturnal. As you found, it eats plum tree leaves as well as the leaves of maple (most common), box elder, alder, elm, ash, and birch as well as wild cherry, apple, and others. The caterpillar is really spectacular looking!
