Melissa's beardie does have mbd quite badly which has weakened the bones - apparently her beardie would only have to turn too sharply to injure her back and that appears to be what has happened so it's a fracture caused by mbd.
I would advise seeing a vet if yours has a similar lump because it may be the vet could do something for the fracture (each one is different) or it may be that your beardie has serious mbd like Melissa's and needs calcium and D3 treatment. I don't think assuming it's just a break you don't need to do anything about because that's what's happened to ub003's beardie is a good idea at all.
I believe any lump like that should be checked. My Sydney's started out smaller then got bigger. Now it is extremely noticable and the vet's not sure she will pull through this. So, please make sure you get him checked out.
my beardie had the same problem, she fractured her spine somehow, possibly jumping off her logs chasing crickets. Luckily, like ub003 it hasnt seem to affect her.
How did her bones look on the xray? maybe she's having problems absorbing calcium?