B737 FLT CREW REPORTS ANOMALOUS GPWS WARNING DURING RADAR VECTORS FOR A VISUAL APPROACH TO RWY 28R AT BOI.

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WE RECEIVED A VISUAL APCH CLRNC FROM BOI APCH AROUND 20 MI FROM THE FIELD AND AT 8000 FT MSL. CLRNC GIVEN WAS 'CLRED VISUAL APCH RWY 28R; MAINTAIN AT OR ABOVE 4500 FT MSL UNTIL 5 MI.' DSNDING THROUGH 4600 FT MSL AT APPROX 6 MI FROM THE RWY AND 1 MI S OF EXTENDED RWY CTRLINE; WE RECEIVED A 'TERRAIN; TERRAIN' WARNING FROM THE GPWS. I LEVELED OFF AT 4550 FT MSL AND NOTICED YELLOW; BOXED TERRAIN ON THE MAP DISPLAY DIRECTLY IN FRONT OF AND AROUND OUR POS. THE RADIO ALTIMETER SHOWED APPROX 1600 FT AGL. THOUGH IT WAS DARK WITH LIMITED GND VISIBILITY; WE COULD SEE THE OUTLINE OF TERRAIN TO THE R OF THE ACFT WITH WHICH WE HAD ADEQUATE ALT CLRNC. WE RECEIVED NO FURTHER AURAL WARNING FROM THE GPWS AND I ELECTED TO CLB APPROX 200 FT. THE CAPT QUERIED APCH CTL ABOUT OUR ALT VERSUS THE MVA; AND APCH REITERATED THAT 'AT OR ABOVE 4500 FT MSL TO 5 MI' WAS THE CORRECT CLRNC. THE YELLOW; 'BOXED' TERRAIN ON THE MAP DISPLAY DISAPPEARED AFTER CLBING APPROX 150; AND AT THAT POINT I ELECTED TO CONFIGURE THE ACFT AND THE SUBSEQUENT LNDG AT BOI WAS UNEVENTFUL. THE CAPT AND I DISCUSSED THE EVENT UPON LNDG AND FELT WE SHOULD SUBMIT A RPT DUE TO THE POSSIBILITY OF ANOTHER CREW RECEIVING THIS WARNING WHILE PERFORMING A VISUAL APCH INTO BOI. NOT SURE IF GPWS DATABASE WAS OFF OR IF THE ALT GIVEN WAS APPROPRIATE FOR THE APCH.