It's not about solid bone. Doing soft tissue damage with a punch should be the last thing you're thinking about when you're defending your life.
What you should worry about is concussion to the cranium. Rattle the head well enough and your opponent will lose consciousness. The biggest risk about striking the jaw is that they guy you're hitting will be quick enough to roll away with it. Otherwise, the jaw is a perfectly good target to aim for.
Here's a little quote (not the best source, but at least in this case, factual)
"the button" "A well timed strike to the jaw either 'on the button' (chin) or to the side could be the equalizer in determining whether you are a victim or a survivor of a street encounter. Again, Hung Suen Wing Chun employs the strikes with the fist, palm or elbow to achieve this end result. The fist could encompass any rear cross or lead or rear hook or uppercut. The palm strike might be made with the heel or blade of the palm. Even the elbow can be used to strike the jaw in several different ways � including upward or horizontal. The Hung Suen �Iron Palm� hand conditioning training drills ensure that any well timed strike to the jaw puts your opponent `to sleep'."
Of course, your 2 years of judo would have taught you this concept... And if that didn't, a couple episodes of UFC will.
