Preserving Floyd: Canning Essentials - Canning Tomatoes and Acidity

Yesterday I got an inquiry about canning tomatoes and acidity.  This is a topic I hear a lot in conversations with home canners.  People tend to think of tomatoes as a highly acidic fruit/vegetable when in fact they most definitely are not.  Tomatoes ride the dividing line between high acid and low acid and there is no way for the home canner to know which side of the line the tomatoes they are working with that day fall on.  Some sources say the lower acid tomato is a result of breeding a tomato for sweetness and palatability.  But if sweetness indicates low acid then there are plenty of sweet heirlooms out there that must be low acid.  My personal theory is better testing methods show that tomatoes have been a fairly low acid fruit all along.