Today's N.Y. Times has a nice article on conventional wisdom about running and how it isn't exactly right. The conventional wisdom is that the best temperature is 55 degrees and the best weather is overcast. Also, drink plenty of fluids because you are more likely to cramp up if you are dehydrated. Turns out the optimal temperature is lower, and for elite runners, a rise in temperature has an extremely minor effect. Also, it turns out that dehydration has nothing to do with muscle cramping, and it also has nothing to do with another common problem of collapsing at the end of races. (I doubt being dehydrated is good for you, but it doesn't have anything to do with the problems normally ascribed to it in running.)
I really wish newspapers covered such science more frequently.