I don't backpack in the summer here in the Ozarks, but I do canoe camp. I just use a synthetic sleeping bag, probably a 50-degree bag. I sleep in shorts and a T-shirt, and almost always go from using the bag as a quilt to zipping it up during the night. It doesn't take much to chill you, but with a good bag you can probably get down into the upper 50s, maybe.
Everybody has their preferences, but I recommend underquilts. Go check over at Hammock Forums where they cover everything hammock related.