Stepheno the buffer on the 20D or any other camera for that matter is just a area where the data is held temporarily until it has been transferred to the CF card. It is not meant to hold data for any amount of time and in fact is fairly limited as to its capacity. If your buffer fills when you are shooting multiple shots (in burst mode) then the camera will prevent you from taking any further shots until some data has been written to your card. In modern cameras we see less and less of this as the speeds of data transfer increases however in this case none of this will be of any comfort to you as there was no card in your camera.
As the others has already said, unfortunately all your photos from today are gone forever, its not a nice feeling but its a thing that many have done before you, and many will still fall victim off. I, like the others highly recommend changing the setting to prevent the camera shooting without a card being present; it wont help you now but will stop the same thing happening in the future.