The site more or less spoils every subplot and from what I can tell it's fairly accurate (aside from a few things which might also have been caused by late casting eg the rejoining of Halle Berry). The writer of the article points out that he assembled it all from press releases etc but fyi, the guys at Pajiba do have reliable sources; they actually broke news about several of the cast members and subplots before any casting was finalized or roles announced.
As they said, it might not be 100% accurate but it should be close. Behind the cut you will find some info about Zac's subplot - it's spoilery but only a short paragraph, so if you read the previous announcements it's not going to be a huge surprise. If you want to know the plots for all cast members, you can do that directly at their site here.
Michelle Pfieffer [sic]: Pfeiffer plays a bitter, put-upon secretary at the recording studio where Jon Bon Jovi is recording. However, she has a run of bad luck (she’s nearly ran over by a cab) and decides to quit her job. She enrolls the assistance of Zac Efron, who is a bike courier that routinely delivers to the studio. He helps her tackle all of her New Year’s Resolutions from the previous year before midnight. Pfieffer [sic] and Efron end up becoming good friends over the course of the evening.Source