I suppose it was only a matter of time.
I think some of them are aphids, although I haven't found a good picture yet. The majority of them are the exact same shade of green as the tomato plants. I think there are some whiteflies, too, but when I went out to take pictures they were gone.
Now I need to decide what to do. I'd like to avoid chemicals if possible, so if there's an organic suggestion I'll go with that first. I'll also go over the plants and hand-squash as many as possible. Spraying with water is supposed to help with whiteflies, so I'll try that too.
It seems only the tomato plants are affected. I looked over the peppers and didn't see anything amiss. I already needed to go to the garden store to get taller stakes; pest control is one more item on the list.