As a follow-up to Al's FanPost from September 23, I found this article in the Sun-Times today.
Looks like the Wrigleyville bars did agree to stop sales after the 7th inning in any potential clinching game in which the Cubs are home.
Sales will reopen approximately 1 hour after the 7th inning and will reopen if the games go into extra innings.
So, keep that in mind if you're heading to the Wrigleyville area during a potential clincher. October 7 (Game 5 of the NLDS) would be the first test of the rules.
Sun-Times: "Wrigleyville bars agree to stop serving alcohol after 7th inning"