Many of us have heard that if you soak a tooth in soda for a few days it will begin to soften. Well, it isn't true, but it doesn't mean you can drink all the soda you want without it causing tooth problems. While soda contains citric and phosphoric acids, so do plenty of other foods and beverages.
Grapefruit juice, for example, is even more acidic than Coke. However, usually your saliva can neutralize these acids without a problem. Not only that, but your teeth get extra protection from their enamel coating. This doesn't mean you can drink all the soda you want; if you nurse a soda all day long, or drink it when you're dehydrated and your mouth is dry, you might run into long term problems.
But if your mouth is healthy, the thing you need to be concerned about isn't acid so much as sugar. Bacterias that live in the mouth ingests the sugar the soda leaves behind, producing acids that eventually destroy tooth enamel and lead to tooth decay.