I first heard about the health detriments/benefits of acid/alkaline foods probably 10 years ago now. When I first heard it, I was very skeptical. It didn't make any sense to me. Nutritionists were putting things like oranges and tomatoes on the alkaline side, but I know full well that oranges and tomatoes are very acidic foods.

Believers tried to convince me that the acidity of the food itself is not what matters, but the "net acid load" it produces in your body after it has gone through all of it's reactions. So in theory, then, foods which are acidic could have a net alkaline load to your body.
[EDIT: This turns to to be true, presuming the metabolic end products are the same as the ash end products from burning]

Yet I remain skeptical of the inference that acid/alkaline properties determine health or disease effects. I know that our bodies operate within very narrow pH tolerances. I know that holding your breath acidifies your blood, and that makes you pass out... the acidification is not sustainable. I know that stomach acid is tremendously acidic, and that this level of acid would seemingly overwhelm any acid/alkaline properties of food.

Well I have found a reputable source that agrees with me: Acid/Alkaline Theory of Disease Is Nonsense

I have also discovered that the whole acid/alkaline food idea started in the 1920s: Hay Diet

And wikipedia's page on the Alkaline diet seems to indicate that the only substantiated benefit is that your body excretes less calcium, and thus your bones probably do not deteriorate as quickly. So it's a good diet if you're worried about oesteoperosis. And yet proponents claim it cures everything that cannot live in an alkaline environment, such as cancer, when in fact the diet does not change the acidity of your body cells at all!! (and none of your cells could survive in an alkaline environment).