I asked an Italian translator and he said that Dedacciai is really not a word in the Italian language but rather a made up name. So he said its pronounced like Dada-Chi. The chi is pronouned like the chi in the word Chinese. He's not a cyclist so that's his educated guess.
Another made up name is Cervelo. Its suppose to be a combination of the Italian word for brain and the French for bike. But most people pronounce it as if it were Spanish. If it were pronounced in Italian the Cer would sound like Cher.