LEOSA, Law Enforcement Officers Safety Act, was passed in 2004 and allows on-duty, retired, or separated law enforcement officers to carry a concealed firearm in any jurisdiction in the United States with some restrictions. Following are links to articles explaining the law, both are worth reading. One article worth reading is in the NRA site explaining how LEOSA does not exempt you from certain gun free zones. For example, State and Local CCW permits or licenses can exempt you from the 1000 foot GFSZ but LEOSA permit does not. A good example is San Francisco.

Wikipedia article

NRA Information