Major League Baseball has told its 30 teams they must implement security screening for fans by then, either with hand-held metal detection or walk-through magnetometers. MLB spokesman Michael Teevan says "This procedure, which results from MLB's continuing work with the Department of Homeland Security to standardize security practices across the game, will be in addition to bag checks."
I'm speculating, of course, but I'd guess that Wrigley Field will use hand-held metal detectors, because the expense of installing the walk-through kind (as you'd have in airports) would be too expensive for the Cubs.
It's unfortunate that we as a society have gotten to the point where these sorts of things are necessary, but that appears to be the way of life we're heading toward, as the second decade of the 21st Century nears its midpoint. (Can you believe that? I'm still trying to get used to it being 2000!)
I, for one, am glad they are doing this before there's some sort of terrible incident at a MLB ballpark.