I forwarded this thread to my brother, an engine builder extrodinar, and here is what he said;
I'm gunna pass on this one (as far as posting), too many variables and the guys have already said the obvious. Sounds like a combination of improper valve train geometry and possibly piston contact - the exhaust valve is really strong down at the valve head but not so strong up the stem, may have hit the piston and loaded the stem up high to cause it to crack, then fail - the damage to the valve didn't happen at idle! Just happen to come unglued at idle.