"Mount Desert Islands" comes it's îles du mont Désert. Frenchman, Samuel de Champlain, who made the first important contribution to the historical record of Mount Desert Island. He led the expedition that landed on Mount Desert on September 5, 1604 and wrote in his journal, "The mountain summits are all bare and rocky..... I name it Isles des Monts Desert."
Although it can be pronounce like a hot expanse of sand or like the treat that comes after dinner, the locals call it "Mount Dessert Island" like the treat.
"In Maine, you're taught to pronounce, "mount dessert island", although they can still tell that you're up from Boston or New York for the summer."
- Correction to the above quote found on the Internet - It's down not up. More on that later.