Interesting article from InformationWeek called Are Background Checks Necessary For IT Workers? about the UBS PaineWebber administrator severely disrupting their computers. My answer is to their hypothetical question is YES!
We recommend investigating the background for administrators handling mission critical systems. From a recent Forrester survey of people responsible for server security, only 35% do additional background checks on their system administrators. These are people handling some of your most important resources! The small upfront cost of a background investigation is a very tangible step to take to reduce insider attack risk.