Actually, if you read the neem oil thread in the "What Doesn't Fit Elsewhere" thread you'll discover some very useful information on Neem Oil including (if memory serves) the MSDS for Azardachtin(sp?); the active ingredient in Neem Oil,
What you'll discover is that Neem Oil inhibits the development of winged insects and you'll also learn that North America's Native Honeybee population is under a great amount of stress. Especially in the east.
Anyway, spray neem oil no more than once a week and employ some other organic techniques such as planting flowers to attract the leaf hoppers predators such as other insects (Lacewings, Minute Pirate Bugs, Ladybugs and so on) and birds.
Also, don't plant your plants in the same area each year. Planting annuals in the same place allows the larvae of the insects to overwinter in that soil and then to reinfect the plants that you replant there.
Furthermore, you can spray your plants with an aerated compost tea weekly to encourage a healthy flora and fauna to grow on the leaves that will aid the plant in fighting any infections, and if any of these leafhoppers lay their eggs on the leaves or stems of your plants, the local Fauna will eat them up right away.
Hope this helps!
Good luck with your tomatoes.