I agree - I have a special tone of voice that I only use when calling the rabbits for a treat, and it takes less than a day for them to learn this. I never use that tone when there is no treat in the offing. I had one rabbit that always came, guaranteed; one that would come when the other one started running towards me and drew his attention to it, and the ones I have now will come if there is nothing more exciting going on. I expect it depends on the greediness of the rabbit.
One of my rabbits has also learnt to jump over a jump, partly in response to a command (although I think it is the tone and rhythm rather than the word) (there is always a treat for him on the other side of the jump).
