A new breed of startups targeted to storefronts is taking advantage of mobile phones to provide ever more precise information about consumer behavior and habits. Turnstyle is one such company using sensors to track users with Wi-Fi enabled smartphones as they go about their day. "Instead of offering a general promotion that may or may not hit a nerve, we can promote specifically to the customer's taste," says Fan Zhang, a restaurateur and Turnstyle user. Such technology is beginning to raise privacy concerns, however.

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