DENVER -- Heather Poli wasn't quite sure how to react when her friend's cellphone informed her she was drunk.

The 27-year-old ad-agency worker had been at a bar here with her buddies. It was late; she was about to catch a ride home. Then a friend pulled out an iPhone, and the gang took turns entering their weights and what they had imbibed into an app called R-U-Buzzed?

Bing!...