MLG FLIGHT CREW EXPERIENCES GPWS TERRAIN WARNING WHILE FLYING NIGHT VISUAL TO RWY 28R AT BOI.

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I WAS THE PF DURING OUR APCH INTO BOI AT NIGHT TO RWY 28R. APCH VECTORED US FOR A R DOWNWIND TO RWY 28R STEPPING US DOWN THROUGH THE MOUNTAINS. THEY TURNED US ON A 10 MI BASE AND TOLD US TO 'DSND TO 4500 FT; MAINTAIN 4500 FT UNTIL ESTABLISHED; CLRED THE VISUAL TO RWY 28R.' WE HAD NOT CALLED THE FIELD IN SIGHT YET AND WE TOLD ATC WE DIDN'T HAVE IT YET. I QUICKLY LOOKED FOR THE RWY AND PICKED IT UP AS WE WERE APCHING FINAL. APCH THEN ASKED IF WE HAD THE FIELD IN SIGHT AND WE TOLD THEM WE DID. THEY REISSUED THE CLRNC FOR THE VISUAL AT 4500 FT UNTIL ESTABLISHED. WHILE ON FINAL; SLIGHTLY R OF COURSE; APPROX 10 MI FROM THE FIELD DSNDING THROUGH 5000 FT; WE RECEIVED A GND PROX WARNING TO PULL UP. I IMMEDIATELY INITIATED THE TERRAIN AVOIDANCE MANEUVER AND CLBED UNTIL THE WARNING STOPPED (APPROX 5500 FT). AFTER CLEAR OF TERRAIN AND REFERRING TO THE TERRAIN DISPLAY; WE RE-ESTABLISHED THE VISUAL APCH AND LANDED UNEVENTFULLY. AFTER LNDG; WE CALLED TWR TO LET THEM KNOW WHAT HAD HAPPENED. THEY SAID THIS HAPPENED QUITE OFTEN AND APCH WAS REVIEWING THEIR PROCS TO TRY TO PREVENT THIS FROM HAPPENING. THEY DID SAY; HOWEVER; THAT ONCE WE ACCEPTED CLRNC FOR THE VISUAL APCH; ATC IS NO LONGER RESPONSIBLE FOR TERRAIN SEPARATION. SEVERAL FACTORS CONTRIBUTED TO THIS ERROR: PRESSURE FROM ATC TO ACCEPT THE VISUAL TO A RWY THAT HAD NO PRECISION APCH; OUR FALSE ASSUMPTION THAT THE ALT CLRNC OF 4500 FT FROM ATC PROVIDED TERRAIN CLRNC; AND NOT SEEING THE TERRAIN DUE TO NIGHT CONDITIONS ALL CONTRIBUTED TO THE EVENT. THANKFULLY; THE GND PROX HELPED TO BREAK THE ERROR CHAIN AND WORKED AS ADVERTISED. DON'T ACCEPT CLRNC FOR THE VISUAL -- ESPECIALLY AT NIGHT -- UNTIL YOU ARE SURE YOU NOT ONLY HAVE THE FIELD IN SIGHT; BUT ALSO THAT YOU ARE AWARE OF ALL TERRAIN/OBSTACLE THREATS. ATC COULD ADVISE THAT THE 4500 FT RESTR IS FOR NOISE ABATEMENT AND DOES NOT PROVIDE TERRAIN CLRNC.