When wireless operators were signing up hoards of new customers, they spent significant time and money increasing their retail store presence across the country. But now that wireless penetration rates are over 90 percent and growth is slowing, some experts argue wireless carrier retail stores are turning into cost centers rather than revenue generators. In fact, consulting firm PRTM estimates major carriers' retail stores each cost about $500,000 per year to operate, which adds up to $1.1 billion annually for AT&T and Verizon. Article (sub. req.)