Acidity is measured primarily by a property called pH. Seven is neutral, above seven is basic (caustic, like lye) and below seven is acidic. This is an inverse concentration based measurement.
While most colas (which also have phosphoric acid - the acid that gives colas their 'bite') have a pH of about 2.8 to 3, the acid that is in your stomach (hydrochloric acid) has a pH that ranges from 1.5 to 3. Therefore, cola often does not change the natural acidity of the stomach.
Interestingly enough, the stomach acid is neutralized by sodium bicarbonate produced in the pancreas to form table salt and carbonic acid - the same acid that carbon dioxide forms in sodas (any water, for that matter). Enzymes break down the carbonic acid (from the stomach acid and cola) into carbon dioxide and water. Eventually, the largest part of that carbon dioxide is exhaled.
Also, vinegar, lemon juice and cranberry juice have lower pH readings (are more acidic) than cola.
When it comes to eroding tooth enamel, according to snopes.com,
Mountain Dew contains citric acid, a substance naturally found in citrus fruits that exists as a powder in its purified, industrialized form. Most citrus sodas mix in the stuff to give drinks their tangy bite, while most colas, such as Coca Cola and Pepsi, incorporate phosphoric acid for the same effect. Consequently, these drinks have a low pH value around 3 (very acidic). Coca Cola, with its dark coloring and non-fruity flavor, may be the soft drink most often compared to battery acid, but in 2004, a well-known study led by dentist J. Anthony von Fraunhofer found that citrus sodas like Mountain Dew and Sprite erode tooth enamel around six times faster than colas.
Catchy catch phrase is catchy.