For some unexplainable reason, the PC version of NBA 2K12 doesn't update the rosters automatically like last year. You have to actually delete any Roster files and restart the game for it to download the latest roster. So to rid you of the small trouble of finding the files and renaming/deleting them, I've created a few .bat files which should do that automatically.
More information about this download:
http://forums.nba-live.com/viewtopic.php?f=143&t=83717