The only way I know how to differentiate this caterpillar from the Tobacco Hornworm is by the color of their “horn”. If black, it is the Tomato variety; if red, it is the Tobacco variety. This tiny little caterpillar is 1/2" long and I’m betting on it being the Tomato Hornworm. Cute little larva:>) The horn could turn color, however, as it progresses through various instars.
They are considered pests of the plants in the nightshade family or Petunias, too. We do pick them off our tomato plants and try to find something wild for them to chow down on, like nightshade. More often than not we see the eggs of a Braconidae wasp on the more grown up caterpillars, so many do not survive to adulthood in our area.
Here is a bit of a write up on them:
Taken with a Canon T4i and Tamron Sp 90 mm F/2.8 1:1 Macro lens, hand held and done by an amateur (brand new to DSLR as of April 2013).