P.S. I apologize for not submitting photos, but I've tried several browsers, including Netscape and IE for Mac, but have not been able to upload any photos.

Well, with just a tiny am't of digging (online, that is), I've answered my own question. I have radial cracking on my Mariana's Peace tomatoes and concentric cracking on my Black Krim's. This is due, I read, to environmental stress. The temperature here has been fluctuating from over 100 degrees F to less than 90, and so on. And the plants went several days w/o watering, due to a malfunctioning drip system that I did not catch. So I've been watering every morning (while it's still cool) but not slowly or deeply, just enough to last till the next morning.

This is what I found when I typed "tomatoes cracking" into Google.com:

Growth Cracks
Two types of growth cracks affect the stem end of tomatoes: concentric and radial. Concentric cracking produces circular cracks around the stem end of the fruit. Radial cracks spread outward from the stem scar. These cracks typically appear as the fruit matures. Growth cracks often appear when conditions drastically change the rate of growth, such as wide fluctuations in temperature and moisture. Dry weather followed by heavy rains causes radial cracking in many tomato cultivars. Cultivars vary in their ability to withstand cracking depending on the strength and "stretchability" of the skin. Very susceptible cultivars will crack while still green, those that are somewhat resistant often donâÂÂt crack until the fruit has reached the breaker stage. The earlier the fruit cracks, the deeper the cracks become. High nitrogen and low potassium are often implicated in fruit cracking and therefore, proper plant nutrition and adequate, regular irrigation will reduce the likelihood of growth cracks.

Source: http://wihort.uwex.edu/fruitveggies/TomatoDisorders.htm (accessed 19 Aug 2008)

NOTE: Overwatering was also noted on a different website as a possible cause of tomato cracking.

So I need to check for high nitrogen and low potassium and get the drip system fixed so a daily deep watering is being given to the plants.

Tomatoes like their world to be steady. A constant flow of water (not too much - not too little) and they want this ALL the time. They want a constant supply of sunlight and a constant supply of minerals, etc.....etc.

Anytime this "Constant" is warped or broken, tomatoes undergo STRESS. Too little water, and the skins lose some elasticity. When extra water is brought back, they drink it up so fast that the skins can't expand quickly enough and so they crack. You did good on the research.

Now, having said that, Some tomatoes are prone to cracking, especially around the stem and shoulders area. Seems to be genetics or something.

Anyhow, hope you get your drip system fixed quickly.