Scaphoideus sp..
Size: approx. 4-5 mm. Photo: 04/29/12, 11 pm.

The genus Scaphoideus has 54 species in North America. They transmit the phytoplasma that causes elm yellows. An expert (see link) pointed out that "the toothlike mark on the middle of the head is characteristic" for this genus. The small photo below was taken on 06/21/12.  

bug guide (large photo):
bug guide (small photo):