Hey Tim, thanks for the photo.

From your picture it appears as if the leaf is losing its green color or almost turning yellow in the spaces between the the veins. This most likely means that your plant has tomato mosaic virus. Some of the symptoms include the diagnostic lose of color between the veins giving it a mosaic pattern, leaf roll and lose of fruit yield.

Unfortunately there is not treatment for tomato mosaic virus. Some ways to prevent the spread of the virus can be found here http://tomatodiseasehelp.com/mosaic-v...

Useful Links:
http://www.clemson.edu/extension/hgic...
http://www.clemson.edu/extension/hgic...
http://www.colostate.edu/Depts/CoopEx...
http://www.extension.umn.edu/garden/y...