Dear Monica, I love your show! Per your recommendation, I bought the SodaStream machine. When I brought it home and explained my new plan to minimize my carbon footprint, my wife said "That's great, but don't you know soda causes tooth decay?" Does carbonated water alone have any effect on tooth enamel?
Most soft drinks are hard on tooth enamel because they are very acidic. The pH of cola, for example, is about 3.0. (Vinegar is about 2.5.) To answer your question, I tested the pH of some plain water, then carbonated it with the SodaStream machine, and retested it. The pH was unchanged! Both were about 6.5, or close to neutral.
From my little experiment, I would conclude that carbonated water is no rougher on your tooth enamel than plain water. So, enjoy your SodaStream machine in good (dental) health!