Online reservations are now available for Disney restaurants at the following address http://disneyworld.disney.go.com/restaurants/
I know, I know its about time too!!! I was using it yesterday picking out places to eat and I have to say that I was very impressed. It is very easy to use, efficient and informative. You select the day and time that you would like to eat in a particular restaurant and it shows you the availability. If your desired restaurant is full it offers other available restaurants in local to your first choice. Also if you have dietary restrictions you can state them easily on your reservation. Which will come in useful with my fatal allergy to tree nuts. (I'm not kidding). I will certainly be using this way of booking next time I head out to the World as it saves hanging on the phone being charged international rates just to hear "I'm sorry sir Le Celliers is fully booked"!
Have a magical day!