B727 FLC DSNDS BELOW ASSIGNED ALT WHILE LOOKING FOR TFC INDICATED ON TCASII.

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ON A SCHEDULED FLT FROM STL TO LAS; WE WERE CLRED; PLT'S DISCRETION; TO DSND TO FL240. AT 90 DME FROM LAS; FO BEGAN HIS DSCNT AND I RPTED LEAVING FL310. AT ABOUT FL260; I SAW A BOGIE ON TCASII AT 11 O'CLOCK ABOUT 12 MI; 3000 FT LOW. OUR ALTS WERE CLOSING BUT IT APPEARED THAT HIS TRAJECTORY WOULD TAKE HIM SLIGHTLY L OF OUR COURSE; SO I WAS NOT CONCERNED. SHORTLY LATER; AT ABOUT FL255; THE TCASII GAVE US A TA. I CHKED THE TCASII WHERE THE BOGIE WAS NOW AT 10:30 O'CLOCK AND ABOUT 5 MI. I IMMEDIATELY LOOKED OUT THE WINDOW AND BEGAN A RAPID SCAN FOR TFC. I REPEATEDLY CHKED TCASII DISPLAY AND OUT THE WINDOW; BUT NEVER SAW THE TFC. I KEPT THIS UP UNTIL THE BOGIE PASSED OUR 9 O'CLOCK POS. I WAS JUST RESUMING MY NORMAL INST SCAN WHEN ATC ASKED US WHAT OUR ALT WAS. I CHKED; SAW THAT IT WAS FL232 AND RPTED THAT TO ATC. HE THEN CLRED US FOR FURTHER DSCNT. THIS INCIDENT HAD SEVERAL CONTRIBUTING FACTORS: 1) THE FE WAS OCCUPIED COPYING ATIS INFO WHICH HAD JUST BECOME LEGIBLE. 2) FO HAD PLANNED HIS DSCNT POINT BASED ON AN OPTIMUM DSCNT PROFILE FOR LNDG; AND WAS CLRLY ANTICIPATING FURTHER DSCNT CLRNC. 3) AT 1000 FT ABOVE OUR ASSIGNED ALT; WHERE I SHOULD HAVE MADE A CALLOUT; I WAS BUSY LOOKING FOR TFC. 4) ATC NEVER MENTIONED THE TFC; ALTHOUGH IN RETROSPECT; IT'S OBVIOUS THAT HE WAS WAITING FOR THAT TFC TO PASS BEFORE ISSUING FURTHER DSCNT CLRNC. 5) TCASII; WHICH IS GENERALLY OF GREAT VALUE TO FLCS; THIS TIME CONTRIBUTED TO A LOSS OF SEPARATION BY DISTRACTING ME ENOUGH TO INTERFERE WITH NORMAL COCKPIT PROCS.