We opened the hour with our guest David Little, of The Little Organic Farm out of Petaluma.   David spoke to us about dry farming tomatoes and potatoes, which he does -- commercially -- for sale at Farmers' markets in San Francisco and several other places in the Bay Area.   Dry Farming is much what it sounds like: after initial planting, the crops are watered little if at all; the result is more hardy, concentrated, flavorful vegetables, all while conserving water.