In my preview on Wednesday all I got to look at was "NBA's Greatest" and the play-now mode of NBA 2K12. So I can't tell you anything about an "Online Association" mode, other than the fact the game's achievement list hints it will have one.
Xbox360Achievements.org, hawkeyed for this sort of thing, found and posted the list, and Pasta Padre picked it apart for some pretty juicy tidbits. Achievements for acquiring skill points and Michael Jordan's dunk package speak to a currency system implemented in "My Player," the career mode, through which you'll rank up your baller. There is also an achievement for creating something called a My2K Account, and then also for joining an Online Association.
Online franchise modes, roughly speaking, take the game's franchise and allows you to play out a season with your friends online, including trades, free agency, the draft, all of the player management stuff. It's not confirmed that's what "Online Association" actually means here, but given how that word has a very specific meaning in this game, I don't think 2K Sports would use it in a misleading way.
There is also an achievement called "Lincoln" but it has nothing to do with acquiring DeShawn Stevenson's tattoo package.
NBA 2K12 Achievements Reveal New Features [Pasta Padre]