I’m not a lawyer, but just looking at the industry it’s easy to see a few guidelines. As long as you aren’t using their actual name and/or physical likeness, you generally are okay. For example, the NCAA and EA just lost the lawsuit filed by Ed O’Bannon regarding using players likenesses, because they were using the players faces.
Games like World of Warcraft on the other hand, use parodies like “Haris Pilton” (http://www.wowhead.com/npc=18756/haris-pilton), a blonde female blood elf who sells fashionable bags. Where you get into a grey area is GTA V with Lindsay Lohan type situations, because you have to remember, in the US all you need is a lawyer willing to file a lawsuit to get the process started, no matter how silly the lawsuit may be.
So if you aim for the Blizzard approach, you’ll be fine, if you aim for the EA or Rockstar Approach, you could find yourself in some hot water down the road. Frankly, I’d concentrate on making a fun game first before worrying about using parody celebrities.