Tramea sp. (Saddlebag).
Size: approx. 50-60 mm. Photo: 08/11/12, 9 am.

This skimmer belongs to the Saddlebags (Tramea sp.) also known as Dancing Gliders. They typically have colored bases to their otherwise translucent hindwings. There are seven species in the US. So far the best guess for this specimen is Teneral Male Black Saddlebags (T. lacerata).  

bug guide (these photos):