Depends on the program/hospital. We recently had a good thread going dealing with precisely this topic (ie, who runs Traumas).
At this facility, Level 3 traumas (ie, minor ones) are staffed by the Surgical Chief on call for Trauma and the ED attending. All Level 2 and above traumas are staffed by the Trauma Team (which is a Surgical rotation; we generally have 1 anesthesia intern on service with us, several surgery residents), including the Trauma Chief. Thus, Traumas here are run by the Surgery Department. The ED attendings either hang around and watch or just leave the Trauma Bay and return to the ED and handle the patients there. They are not involved in any other way except to write a brief note about the Trauma Call/triage. If the Trauma is especially bad, the Chief is in the thick of things, but generally the call is run by the junior HO (ie, we do the Primary and Secondary Surveys, draw blood, insert catheters, etc.) with the Chief directing our actions, deciding what needs to be done, etc.