According to this recent post at Chicago Breaking News, the guy who did it called police today:
A Bartlett man believed to have tossed a beer on a Philadelphia Phillies outfielder during last night's Cubs game has turned himself in to Chicago police detectives.
The 21-year-old man contacted police and said he wanted to come in to talk about the alleged battery. A friend drove him to Chicago this afternoon to meet with detectives.
There's a good photo on that link of some of the people involved.