If you're a restaurant owner, even one who wants to serve organic food, you don't actually have to be certified organic unless you'd like to be. Current National Organic Program (NOP) regulations state that retail operations, including restaurants, are usually considered excluded from certification. Due to this, among other issues, very few restaurants are certified.
However, the above doesn't mean you shouldn't at least consider certification. If you do own a restaurant you may choose to become voluntarily certified organic. Obviously certification will set you apart from other "natural" dining establishments and it'll show your customers that you not only value organics, but uphold your restaurant and food to specific organic standards. On top of your own values, note that consumers are often willing to pay more for organic and/or "green" food.
To learn more about going organic in your own restaurant, check out the links below.