Hi, I had the same problem, one of the garden pro's gave me some very good advise, and I saved it to my clippings. Just click on my "my page" and my clippings, and it is the first 2 clippings there. The link below has very discriptive pictures that helped me determine it was probably early blight with some possible bacterial spots. I think my yellowing leaves have already slowed down with just one dose of the baking soda spray. Be sure to read both of the clippings, and remember for next season to spray every 7-10 days to prevent the problem. Good luck!

http://aggie-horticulture.tamu.edu/tomatoproblemsolver/index.html