I haven't heard of excessive licking in rabbits, but I do know that with other animals-dogs, cats, horses, etc. that this type of behavior is usually because of a nutritional deficiency. They will lick (and in some cases eat) random objects in order to try and obtain certain nutrients that they are lacking in with their normaldiet (or they may have anabnormal digestive system that can't take incertain nutrients).What do you feed your rabbit exactly?
In rare cases, it has been linked to psychological issues. For example, when cats are taken away from their mother before 6 weeks of age, they candevelop suckling and licking problems that stay with them through out their adult lives.
I've also seen a dog with severe excessive licking issues. He had damaged his nasal cavity at some point in his life, and so to make up for his loss of smell, he would lick EVERYTHING (including the air around him) to try and take in the scents.
So, with your rabbit, it could quite possibly be a number of problems. You may want to consult a vet for their opinion later on.