Crematogasters on cynipid gall - Crematogaster cerasi -
Madequechan, Nantucket, Nantucket County, Massachusetts, USA September 8, 2011 Size: ~2.5 mm
The gall was on scrub oak. The only eastern honeydew-secreting galls I know of are 'pip' galls that grow on acorns, like this one:
but this one seemed to be coming out of a leaf bud, so I'm not sure what it might be.