My wife came down with a bad case of "Disney Rash" during a trip to WDW in April 2009. On the morning of our third day of our trip, she noticed a a little bit of redness on her lower legs above her sockline. By that night, the rash had turned a deep red color and had extended all the way from her sockline to half way up her calf. She also was experiencing a burning sensation in her lower legs. We went to the Celebration Hospital Emergency Room where she was diagnosed with "Disney Rash." The physician said that the Disney Rash was caused by a combination of heat and sunlight reflecting off the pavement, resulting in the bursting of tiny blood vessels right below the skin. Although it can affect anyone, people with circulatory problems or varicose veins are more prone to these symptoms. My wife was advised to keep her feet elevated, keep her legs covered and out of the sunlight, and make sure that she sunscreen on her legs whenever they were exposed to the sun. For the rest of our trip, my wife used a wheel chair and we bought a beach towel to cover her legs. The physician at Celebration Hospital said that the rash would resolve within two weeks, and she was right. However, she also said that in extreme cases, the bursting blood vessels can exacerbate circulatory problems and result in dangerous blood clots.
By the way, there was no "rash" on the surface of the skin. The bursting blood vessels were just under the outer layer of skin.
For those interested, here is a photo we took while my wife was waiting to see the ER physician. Notice how the Disney Rash starts right above her sockline.