According to KSC's launch control, "Fueling was halted after the leak was detected near the Ground Umbilical Carrier Plate, which (is) attached to the external tank at its intertank area. The line leads from the GUCP back to the launch pad and to the "flare stack" where vented gaseous hydrogen is burned off.
Engineers will begin to access the area (image above) late today, and NASA managers will meet to target their options for a new launch date soon after. A similar leak occurred in March on the STS-119 Discovery launch.
NASA may not be able to launch until July 12th, due to a pair of unmanned Cape launches this month, and the station's attitude to the Sun, according to mission management.