I was able to identify it as this moth that we found several weeks ago:It is a Pandorus Sphinx moth (Eumorpha pandorus). My book says that the caterpillar (about 3.5 inches long!) is "frequently encountered while it is wandering in search of a pupaton site." It also says that grape is a common host plant for it, so I left it by the grapevine in front of the cabin in the hopes that it would find that a suitable site for its metamorphosis.
Pretty cool, huh?