Susan Purdy is a friend and colleague of mine, and she recently did research on high altitude baking. This is her site. It includes the most up-to-to date and accurate information about High Altitude Baking there is right now: http://highaltitudebaking.com/index.htm See especially http://highaltitudebaking.com/adj_recipes.htm

Get her reference guide, too! ~ Pie in the Sky Successful Baking at High Altitudes : 100 Cakes, Pies, Cookies, Breads, and Pastries Home-tested for Baking at Sea Level, 3,000, 5,000, 7,000, and 10,000 feet (and Anywhere in Between).

The reason for special high altitude cooking and baking instructions is because the boiling point of water changes with altitude. As you go higher, the boiling temperature decreases.

Photo of a cake before and after adjusting for high altitude.