I have heard about some dogs whacking things with their tails and getting them cut, but nothing like actually breaking the tail.
funny you should say that just now --- Kiara just went to the vet and he suspects she broke her tail! (See the somethings-wrong-with-Kiara thread)
My sister-in-law used to have a bloodhound that would split his tail open on stuff all the time - in the house, in the car- she said tere would be blook spattered everywhere. But since she's a nurse, maybe she didn't mind? She said she sure loved that big ol'bloodhound!