I copied and pasted this from http://www.bfbbenefit.com/index.php?id=44
There is a ton of information regarding fiduciaries on this site-

“Generally a person can become a fiduciary in one of two ways. First, when they are expressly appointed a fiduciary, usually as the plan administrator, plan trustee, or member of an administrative or investment committee. Second, when they perform a duty or function that is deemed to be a fiduciary function. Adding to the complexity, however, a fiduciary relationship also may be legally inferred from circumstances. In other words you don’t have to be formally appointed to become a fiduciary. Your actions regarding a retirement plan are more determinative than the name of your position.”