As is the unfortunate custom, the fan who caught it threw it back on the field. There's a lot of pressure to do this, and I think you all know how I feel about this "tradition." (If you don't, I'm very much against it, and keep a "throwback" ball in my backpack so I can keep the actual HR ball if I ever catch one from a visiting player.)
Via Reddit, we have the story of what happened to that fan after he threw Zimmerman's ball back. The fan, who, according to the Reddit post was a Red Sox fan visiting Chicago (thus the reference to "Sox" below is Red Sox, not White Sox) had just thrown the ball back when this happened (typos and spelling as in the original post):
Minutes later somebody walked up to me from Wrigley and said to me "You made that catch? Nice. I'm gonna try and get that ball for you man, I'll be back later." 5 minutes later or so he walked back with a baseball, I'm not sure still if it's the one I caught or if it's just a ball used in BP or something but there's a good chance it is the ball I caught. He also gave me hat too, not an authentic one but still, free is good. I thought the praise was over, but about 15 minutes later somebody comes up to me. He says "Did you throw the real ball back?" "Yeah" i replied "Well then sir, you're a true cubs fan." (Ironically I'm a sox fan but i didn't say anything..) He then hands me a baseball and says "The second baseman Darwin Barney saw you made that catch and felt bad that you had to throw it back. He signed this and wanted me to give it you."
Say what you want about Darwin Barney's 2013 season performance -- and it was pretty bad -- clearly, he's a real good guy. Props to him for making this fan's day.
The comments to the Reddit post are interesting, too, including this one:
From the Indians webpage on fan conduct, "Home run or foul balls hit into the stands may not be thrown back onto the playing field.", and I know I've heard plenty of other stadiums also have this rule, but a quick google search didn't return much else. Fans should be aware that despite the "tradition" of it, throwing HR balls back will get you kicked out of some stadiums, if not a majority of 'em.
And in my view, it's time to stop this lame "tradition" at Wrigley Field, too.