Oakleaf- It is possible to die of altitude sickness but I think it is more danger for hardcore climbers.

Very good point about heat, I would feel it is the biggest natural enemy for cyclists. Even as an avid cyclist and native Texan I have had heat exhaustion. I had all the symptoms above and it was not pretty.

I know at summer rides here in Texas we often see people who ride frequently sagging or worse being hooked up to IVs for fluids, taken by ambulance to be tended to for heat related illness. Be careful out there guys and learn what works for you.