Agallia constricta.
Size: approx. 3-4 mm. Photo: 05/28/12, 10 pm.

Agallia constricta is one of 14 North American species in this genus. The nymphs in this genus have characteristic ear-like nodules between their eyes. A very similar species is A. quadripunctata which has wider wings than A. constricta. The number of dark spots seems to be a less important feature.  
bug guide (this photo):