I am pretty sure this is an imperial moth. I notice some of us have either been posting some of these beauties--well, butterflies mostly, but I always think of moths when I think of butterflies. Anyway, I found Butterflies and Moths of North America and that will be helpful to some of us. The thumbnails of the photos in the gallery are not real great, but most of the time the ones I clicked to enlarge were pretty decent photographs.
And THIS is another place I found with photographs--I don't think it quite as good as the one listed above but it isn't too bad.