http://www.snopes.com/cokelore/tooth.asp
This link, which discusses the validity of the statement that coke can dissolve a tooth, nail or anything else, states that, yes, the acids contained in coke can EVENTUALLY cause breakdown and then dissolve just about anything, HOWEVER, it is stated that the same acids (and same amount of them) are present in plenty of other things that we eat and drink on a daily basis (another example given was orange juice).
There is also the fact that you don't hold liquids you drink in your mouth for extended periods of time or scrub the liquid into your teeth (as you would if you were either soaking something in coke or scrubbing something with it), so the exposure is minimal.
Obviously, anything you eat or drink with sugar or acids is going to have some kind of effect on your teeth, and something with MORE sugar and acid is going to have more effect, but this is why you're encouraged to brush after meals.
If you're concerned about the effect of coke (or any other food/drink) on your teeth, you may want to consider carrying around a small bottle of mouthwash that will help you keep any sugary or acidic residue of your teeth after drinking anything but water.