I had first heard of Ishikava diagram in the Analyse phase of a Six Sigma project, and wondered about its usage in IT because at time I had learnt only about the manufacturing side of the usage. Over time, I used this diagram very effectively in technical support and service desk process improvements. The key benefit of using this diagram was to be able to identify all impacting factors and then eleminate the not so important ones to reach the vital few. (It of course required more than the Ishikawa, but it sure was the starting point). A very practical usage of this diagram/technique is visible while doing RCAs in the Problem Management Process. I am also going to use this to develop a CoD model in near future!
Another good link to read through would be this from isixsigma which has an overview of the Ishikawa