I've done some tests on adding salts and modifying pH of beer, you can read about it in September's BYO magazine. I've found with dark beers, raising the pH a point or two mellows the roastiness but if you do the same to light beers, they become lifeless and, using the wine term, flabby. On the other hand, light beers can be improved by lowering the pH a point or two, making a bright flavored beer out of one that was lifeless. So if I were to try to carbonate using baking soda, I'd add acidity as well, food-grade lactic acid or phosphoric acid, to balance the change in pH of the beer.
