Unfortunately, the classic mobile game of Dope Wars (I played hours of it on my old TI-80) still hasn't been allowed on the App Store -- Apple reportedly wasn't happy with the drug-themed gameplay, and so there are only "sugarcoated" versions there now (including one that is literally about sugar, branded as Candy Wars). But Zynga, makers of Scramble Live, have now released a game called Mafia Wars, which brings the same kind of premise: you're a mob boss, and you buy and use firearms to get money and all sorts of illegal items. The gameplay isn't exactly the same (rather than selling for higher and lower prices, you're "doing jobs" that cash out earned points for money), but Mafia Wars goes a little farther than Dope Wars, too, in that, since the game was originally created on MySpace and Facebook, you can "fight" against other players and win cash from them, too.
Don't expect to see any of Grand Theft Auto's 3D graphics or storyline (for that, you can go try Payback) -- this is a strictly turn-based button pressing simulation. But it's got quite a life of its own on Facebook already, so it's well balanced and fun for a few pick-up-and-play minutes at a time. It's free right now on the App Store, though Zynga says there'll be "premium versions" (whatever that means) that cost from $10 to $40 (whatever that means) available soon.