With the assumption of otherwise healthy plants, the only thing that fits that picture is the physiological condition known as grey wall. I rarely see it, but this would be a really classic example... my bet is that the fruit is dry and crunchy inside, perhaps with dark patches where the vascular tissue runs between the seed locules. I can't find any really good pictures online, but the Seminis/Petoseed/Monsanto book on tomato diseases has a picture very similar to yours.

Nobody knows quite what causes it. Almost everyone agrees that it's caused by plant stress, but the list of factors is as long as my arm: high nitrogen, low potassium, high soil moisture, low soil moisture, high humidity, temperature fluctuations, high light levels, low light levels, small root system... and to top it off, some resources cite that some bacteria, fungi, and possibly TMV may be involved. It's the "kitchen sink" disease for tomatoes. Some varieties are more susceptible than others, but it varies from year to year.

The only advice I can offer is to check your growing conditions: feed them moderately (or not much at all, if you amended heavily this season); give them consistent water, with a few deep waterings per week preferable to watering every day; and if you can't find anything else, maybe try a tonic like Superthrive or compost tea or some such to see whether it balances things. Then pray for sane weather, with reasonable temperatures, lower humidity, and enough (but not blazing) sun.

Best of luck.

--Alison

--Alison