Eustala sp. (Humpbacked Orbweaver).
Size: approx. 8-10 mm. Photo: 09/29/11, 7 pm.

A Humpbacked Orbweaver (Eustala sp.) 13 species in the US. Its web was suspended between two small magnolia trees at a height of about 6'. I found it around 7:30 pm and it had just started rebuilding its orbweb for the night. 
bug guide (this photo):