The explosion in popularity of smart phones has been a boon for wireless carriers, at least when it comes to revenue growth. AT&T sells millions of iPhones and other smart devices each month, luring customers by heavily subsidizing these mini-computers in exchange for signing long-term contracts. But even if this model is helping firms like AT&T achieve vertiginous top-line growth, many wonder whether selling smartphones for less than they cost to make is a winning strategy in the long run.