I'm not completely sure, but according to this map (clicky) there's four - one between Erickson and the Library, one on the corner of Poplar and Center (I think the one you're referring to), one on Commons Circle next to the Theater, and one outside the Admin Tower. I'd assume that the intention is for people to be able to call a cab, name a stop, and the cab agency would know where to send the driver. I kinda doubt they'll have cabs regularly stationed there as the term usually implies.