Sounds like you got a case of the blossom end rot (BER). Go search the tomato disease and pest forum and you'll find lots of threads on the subject. Bottom line: it's usually due to over-watering or not watering regularly, and the problem may go away on it's own as the plant matures. Haven't had the problem myself, so be sure to check out that other forum.

Blossom End Rot? Often caused by uneven or inconsistent watering causing calcium uptake problems.

Don't forget to check out the GardenWeb Tomato forum:

http://forums.gardenweb.com/forums/tomato/

Mark

What soil did you use in your containers? Some potting mixes may require additional calcium in the form of agricultural lime.

I had the same problem last year, my friend called it a blight or a fungus in the soil. I replaced the soil this year with Miracle Grow and was fine.

All my Tomatoes, peppers and cuc's are growing in 5 gallon buckets.

I use a moisture meter to measure the container soil and then figure out if the container needs water or not.
They are cheap and they take all the guess work out of watering.

I use Sta-Green container soil.
Every 14 days I make a soup of Miracle grow and Epsom salts.
1 teaspoon MG and 1 teaspoon Epsom salts per gallon.
I dump about 12 ounces in each container.

Every plant this year is awesome and producing tons of fruit.
Just throwing out some info, hope some of it helps.
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