It looks like a fungal disease to me, maybe Septoria Leaf Spot; it's hard to tell from the picture. Septoria tends to have darker spots than Early Blight, though.

I'd suggest getting some Ortho Garden Disease Control (its active ingredient should be chlorothalonil) and spraying as directed. Chlorothalonil is one of the safer chemicals out there, and doesn't have any real effect on people or pets; just watch that you don't get any into streams or ponds, as it's bad for aquatic critters.

Chlorothalonil (you'll sometimes see us calling it Daconil) has to have a good coating on the leaves to prevent spores from taking hold, so if you do overhead irrigation or get rain, you'll need to reapply. When you bring in fruit, just wash off any residue -- it doesn't absorb into the fruit or plant at all.

--Alison
who is generally against chemical solutions, but finds Garden Disease Control to be pretty harmless and quite effective.