There are already a handful of ballparks with metal detectors. Oakland and Detroit I know of, but there may be others. As of Tuesday Yankee Stadium will join their ranks. All of this is to get ready for 2015, when MLB will require them everywhere. Here is the statement from Yankees COO Lonn Trost:
“Nothing is more important to us at Yankee Stadium than the safety of the fans we serve. To that end, we are committed to the procedures that Major League Baseball and the Department of Homeland Security have asked us to implement. We want our fans to feel safe in Yankee Stadium, and our staff is dedicated to maintaining the highest security standards possible.”
I can’t recall any incidents at ballparks in my lifetime which would have been prevented by metal detectors, but I suppose that’s where we are now as a society.