Under normal conditions, there is no problem because both of these compounds are safe. But when a bottled beverage containing these compounds is exposed to excess light and heat, the benzoate and ascorbic acid can form a substance called benzene.
Exposure to large amounts of benzene is known to cause cancer; however, this exposure is usually from coal burning and gasoline emissions -- not from drinking soft drinks.
The United States Food and Drug Administration (FDA) tested over 200 soft drinks and released a list of benzoate levels found in them. That list is available here on the FDA site. The levels of benzene detected in most of the soft drinks was below five parts per billion, which is considered safe for humans. Plus the manufacturers of the few soft drinks that did make the list reformulated their products.
United States Food and Drug Administration. "Benzene in Soft Drinks." Published April 2006.
United States Food and Drug Administration. "Data on Benzene in Soft Drinks and Other Beverages." Accessed February 9, 2012. http://www.fda.gov/Food/FoodSafety/FoodContaminantsAdulteration/ChemicalContaminants/Benzene/ucm055815.htm.
United States Food and Drug Administration. "Questions and Answers on the Occurrence of Benzene in Soft Drinks and Other Beverages." Accessed February 9, 2012. http://www.fda.gov/Food/FoodSafety/FoodContaminantsAdulteration/ChemicalContaminants/Benzene/ucm055131.htm.