yet another id

[jd]

I took a photo of a truly amazing creature, which I assume is a moth of some 
sort.  It was large (7-inch wingspan).  It was "flexing" its wings laterally 
(not up & down flapping), was it stretching out?  warning me to get away?
In the city of Denver, about a month ago.

photo at: http://www.csd.net/~jdaues/moth/moth.jpg

i suppose it could be a black witch as discussed in a recent thread, depending 
how much the markings can vary.

anybody?