I went out to fertilize and check on my little pretties (container garden) today and saw there was this giant worm, 4-5 inches long, green, fat, horned - exactly the color of my tomato plant. I've included a video of my li'l garden. About a week ago I started noticing my cherry tomatoes were looking a bit shabby and some of the stems eaten off. Then I saw a bird (didn't ID it) atop my tomato plants. I thought the birds were the problem. After googling "giant green worm tomatoes", the answer became plain. This is a Tomato Hornworm Caterpillar that turns into a giant moth eventually. There is a great Yahoo answers thread on the subject, FYI. You can leave it be and let the birds take care of it, pick it off (blick!), or use pesticides, natural BT or natural lime to dust, among a few other things. I think I'll leave it be. I enjoy seeing the birds up close and personal, doing their thing, more than I like cherry tomatoes. :)
Oh, it also said you can plant sweet basil IN with your tomatoes, which I will try next year. I forgot to grow marigolds this year to take care of the aphids....