To me, the stem looks thinner just above the ground.

There was a post a couple of weeks ago (?) about seedlings which seemed to have survived a bout with damping-off; they were narrow in the same place.

bartonmt, when you planted these three in the garden, did you plant them below ground level or at the same level as in their original pots? If the former, I assume my damping-off hypotheses would be incorrect.[Since tomatoes will root anywhere, planting them low makes for larger and stronger root systems.]

My second question is, do you have a dog which might have "watered" the plant?

Clearly dying but the cause could be many things - too much water, too little water, root damage, stem damage, damp-off, root bound, etc. etc. - and no way to know without examining the plant.

Dave

The stem was very weak and thin just above soil line. It also felt kind of soggy right there. Don't really know what this means. Yes, I have a dog, but he seems to prefer to 'water' my hydrangeas.

I think it was probably damping-off. You can read about it at sites like these:
http://gardening.about.com/od/gardenproblems/qt/Damping-Off.htm
http://www.ehow.com/how_9361_prevent-damping-seedlings.html
http://tomclothier.hort.net/page13.html

And here's the old thread that has the photos of a young plant with a narrowed stem:
http://forums.gardenweb.com/forums/load/tomato/msg0416271013922.html

If you didn't start the plants from seed yourself, you can blame the grower.