My son was sick today, and we've watched Episode II twice now. I don't know how I missed it before, but it's right there. They show the helmet getting dented. And not in an Expanded Universe kind of way.
I am now convinced Boba takes Jango's helmet for his own. The helemt clearly gets dented in the arena when the three horned beast bowls him over!
I don't know if this has been discussed before, but I was sure excited noticing it for the first time!