Hemaris thysbe - Hodges#7853 (Hummingbird Clearwing).
Size: approx. 40-50 mm. Photo: 09/11/11, noon.

Once again I initially thought that I am looking at a humingbird but this is a Hummingbird Clearwing Moth (Hemaris thysbe). It was very difficult to take a decent photo of this fast flying and very nervous moth. Its larvae feed on hawthorn, honeysuckle, and viburnum while adults take nectar from deep-throated flowers like these petunias. 

bug guide (these photos):
moth photographers group: