Svend: By the way, thanks for the research. My helmet is of the type that doesn't show the damage. I had already decided to replace it (as you said, cheap insurance), but your research just confirmed the wisdom of that idea.
On the other hand, I've been hit in the head with the backswing of a baseball bat (that one staggered me a bit), I've been hit around the eye by a line drive, I took a fall on my bike last summer that actually split my bike helmet all the way through for 6" or so (that is like your fall, I don't remember a thing about it, did go to the emergency room for CT scan), and I haven't had a problem yet.
On the other hand, my daughter-in-law got run down by a biker while out running and training for the Marine Corps marathon last fall. She broke her collar bone, and did have bleeding on the brain from the impact of the fall. Pretty touchy overnight. To my everlasting amazement, she finished the Marine Corps marathon within the time limit even though the doctors wouldn't let her run more than 1 mile at a time. Run 1, walk 1, run 1, walk 1. Her father had died of cancer last spring, and she and my son ran the marathon as a cancer fund-raiser and in his memory.