I found some of these, too, on a tomato plant in Tustin, California. After some research, I found that they are, indeed, Antianthe expansa, or Keeled Treehopper. What no one has told you, however, is that the critters in your picture are nymphs, not adult insects. I found both nymphs and adult specimens going to town on my friend's tomatoes. For some good color images of both check out:
http://mamba.bio.uci.edu/~pjbryant/biodiv/hemipt/KeeledTreehopper.htm

Identification by Chris Dietrich of Illinois Natural History Survey. He says that this is Neotropical, and just barely makes its way into southern California.

This is a colony of treehoppers, family Membracidae, and you are quite correct in your assessment of the ant behavior. The ants offer "guard duty" in exchange for the honeydew secretions. These are all nymphal membracids apparently. They do little, if any damage to the plants they are on, by the way. Watch and see if they don't communicate with each other by vibrating their bodies if distressed.