I was over at the Desert Museum today and no, to answer Florian's question, nothing new in the way of signage has been added (unless I walked right past it without noticing

) Maybe the thinking is that the existing sign there already contains some cautions (although none, in my view, clear or strong enough.)
I agree with the general idea that sometimes simple and scary is better rather than long and bureacratic. Personally I think Skyline should have a sign that says in big letters "Warning: Do not hike this trail when temperatures exceed 88 degrees. Extreme heatstroke danger!"
I picked 88 degrees--a fairly low temperature--because of course, people would push it. Setting the bar low might mean that when it was 94, people would give it a pass. But I'm afraid I agree with Wildhorse and others than generally, people will ignore signs.
Q: How many therapists does it take to screw in a light bulb? A: Only one, but the light bulb has to want to change ...