It wouldn't surprise me if distributors end up just averaging the added cost to retailers across both diet and non-diet varieties.  So instead of a 12-pack of diet Coke costing the normal $3.50 and regular Coke carrying the additional $2.88 ($6.38 total), that both will just be priced at $4.99 now for marketing purposes and totally undercutting the intent of the (ill-advised) law.