Acronicta sp. (Dagger Moth).
Size: approx. 25 mm. Photo: 10/09/11, 9 pm.

A dagger moth (Acronicta sp.) There are 73 different species of Acronicta in North America. The common name refers to black dagger-shaped markings on the upper forewings, which are not very pronounced on this specimen. Caterpillars of most species have brightly colored, hairy spikes. The caterpillars often feed quite visibly on common foliate trees. 
bug guide (this photo):