This is a gray hairstreak (Strymon melinus). The only time I had a chance to photograph this species was last August in my backyard. If you look closely, you will notice that this individual is missing a small portion of its hindwing, including one of the pair of hair-like tails it had. In a previous post I had a picture of a butterfly with wings in much worse condition.
Beth at Fireforest Blog notes that these butterflies seldom leave their wings open when they are nectaring. Nevertheless, she was able to photograph one with its wings partially open.