I checked on my tomatoes a few days ago and was horrified to discover that many of the tomatoes on my "Red Zebra" plant are rotting from the bottom up. From above they looked perfect. If it hadn't been for the fact that I looked at something lying on the soil, I would never have noticed this problem.
I wondered if it was blight, which would necessitate the immediate removal of the plants - you are supposed to wrap them and put them in the garbage rather than compost them in order to limit the spread of the fungus. The leaves all look fine, however. A friend who lives in normally rainy and cool Oregon (where it was 107 degrees recently!) said that she thinks it's just the result of the incredible amount of rain that we had here this year because something similar has happened to her tomato plants in the past.