The objective of the game is to become the most successful drug dealer in your neighborhood. Don't expect gun fights; you do this by selling high (no pun intended) and buying low.
However, the game isn't interesting only because of its controversial content; the authors claim it's an entirely new concept of mobile gaming called - somewhat jokingly - LBMMOG: Location Based Massively Multiplayer Online Game. This means that you're actually playing against other live players, and everyone's location is real, and determined through iPhone's GPS.
Unfortunately, the game is currently in early, closed beta stage, but, according to the developers, a free beta version will be launched in the next couple of weeks and distributed through the iTunes App Store. I've no doubt that we'll see more and more games which use the iPhone's location and networking capabilities to create massive multiplayer worlds in which you can immerse on the go.