B737 BBJ FLT CREW DURING A VISUAL NIGHT APCH TO RWY 31R DAL RECEIVE A GPWS 'OBSTACLE' ALERT; EXECUTES A GAR AND RETURNS FOR LNDG.

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AT THE END OF A 9 1/2 HR FLT; WE HAD A NIGHT ARR TO DAL. THE WX WAS BENIGN AND VISUAL AND ILS APCHS TO RWY 31L AND RWY 31R WERE IN USE. IT WAS A VERY DARK; HIGH OVCST NIGHT. I ASKED THE CAPT IF HE WANTED THE ILS SINCE WE HAD TO CHK THE AUTOLAND CAPABILITY OF THE AIRPLANE; BUT HE SAID THAT WE COULD STILL COUPLE UP THE AUTOLAND FROM A VISUAL (WHICH WE CAN); SO HE WANTED A VISUAL TO SAVE A FEW MI. WHILE I HAVE A PERSONAL RULE AGAINST NIGHT VISUALS AT UNFAMILIAR ARPTS; I DIDN'T PUT MY FOOT DOWN AND ASKED APCH FOR THE VISUAL. THE CAPT SET UP FOR ABOUT A 5 MI R BASE LEG AND DSNDED TO 1700 FT (ABOUT 500 FT BELOW THE FAF ALT FOR ROSSE; A 5 MI FIX; AND ABOUT 1200 FT AGL). AT THE TIME I THOUGHT IT WAS PRETTY LOW AND A BIT SPORTY BUT ALLOWED HIM TO CONTINUE. AS WE NEARED ROSSE; I STARTED NOTICING THAT THE BUILDINGS OF DOWNTOWN DALLAS ARE LOCATED NEAR ROSSE AND LOOKED VERY CLOSE. WE THEN GOT A GPWS 'OBSTACLE' CAUTION AND THE CAPT TURNED N TO AVOID THE OBSTACLE. HE CONTINUED TO DSND IN THE TURN AS I LOOKED OUT THE WINDOWS. WHEN I LOOKED BACK INSIDE WE WERE AT APPROX 600 FT RA SOMEWHERE SE OF THE ARPT; NOWHERE NEAR THE LOC. I TOLD HIM HE NEEDED TO CLB AND TURN 30 DEGS L TO GET BACK ON PATH. HE MADE SOME COMMENT ABOUT THE OBSTACLE AND I TOLD HIM WE NEEDED TO GO AROUND AS WE WOULD NEVER GET AN AUTOLAND FROM THIS LOW AND CLOSE. HE AGREED AND WE ACCELERATED; CLEANED UP; DECLARED A GAR AND GOT VECTORS BACK TO AN UNEVENTFUL ILS AUTOLAND. I AM A QUALIFIED CAPT AND EXPERIENCED ON THE TYPE. HOWEVER; FOR THIS TEST FLT I WAS SERVING AS SIC/PLT MONITORING. THE PIC/PF WAS A HIGHLY EXPERIENCED TRAINING CAPT SO I ALLOWED HIM MORE LATITUDE THAN I SHOULD HAVE. A SUBSEQUENT REVIEW OF THE CHARTED OBSTACLES SHOW THAT WE MAY HAVE BEEN WITHIN A COUPLE HUNDRED FT VERTLY OF THE PUBLISHED OBSTACLES AND WERE CERTAINLY IN A HIGH RISK LOCATION. I'VE HAD SOME OF MY MOST WORRISOME MOMENTS WHILE FLYING WITH 'HIGHLY EXPERIENCED CAPTS.' IT'S VERY EASY TO LET YOUR GUARD DOWN WHEN FLYING WITH THESE PLTS; BUT THEY DO MAKE MISTAKES AND IT'S UP TO THE ENTIRE CREW TO CATCH THE ERRORS.