B737 LNDG DFW ON VISUAL APCH INITIALLY LINES UP WITH WRONG RWY.

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VECTORED FOR FINAL FOR RWY 17C LOC TUNED IN AND IDENTED. WE WERE A LITTLE HIGH AND FAST; SO I WAS MONITORING THE SPDS FOR FLAP DEPLOYMENT. ON FINAL; WE WERE TOLD THAT ONE DEP WOULD OCCUR PRIOR TO OUR ARR. I GLANCED UP AND SAW AN ACFT TAKE THE RWY. MY HEAD WENT BACK INTO THE COCKPIT TO MONITOR THE SPD FOR A 40 DEG FLAP DEPLOYMENT. AFTER DEPLOYING THE FLAPS AND ANNOUNCING THAT THE CHKLIST WAS COMPLETE; I LOOKED BACK UP. TWR SAID WE WERE CLRED FOR LNDG RWY 17C. THAT'S WHEN MY CAPT NOTICED HE WAS LINED UP ON THE WRONG RWY AND CORRECTED. APPROX ALT WAS 800 FT. CTLR DID NOT STATE THAT THERE WAS ANY ALARM ON HIS PART; THAT IS THE LAST WE HEARD OF IT. IN RETROSPECT; THE FACT THAT I WAS MONITORING THE SPD FOR FLAP DEPLOYMENT DID NOT ALLOW ME THE VISUAL SITUATIONAL AWARENESS THAT SHOULD HAVE BEEN PRESENT. WE WERE ON A VISUAL APCH AND TWR STATED THAT ONE DEP WOULD OCCUR PRIOR TO OUR LNDG. BOTH THE CAPT AND I FOCUSED ON THE RWY THAT HAD AN ACFT PREPARING FOR DEP. I RETURNED TO THE CHKLIST; THE CAPT REMAINED OUTSIDE. WORKLOAD; AND THE PWR OF SUGGESTION OF AN ACFT TAKING THE ACTIVE; MORE THAN LIKELY DREW BOTH OUR EYES MOMENTARILY TO THE WRONG RWY. THE CAPT STATED THAT HE SAW THAT THE LOC WAS OFF AND THAT SHOULD HAVE CLUED HIM IN; BUT THE WORKLOAD AND THE SUGGESTION; ACFT TAKING THE RWY; TOGETHER; PROVIDED A FORMULA TO DRAW OUR ATTN TO THE WRONG RWY. WE CORRECTED THE SIT WITH APPARENTLY NO ILL RESPONSE FROM THE TWR. BUT IN THE FUTURE; EVEN ON VISUALS; IT WOULD BE PRUDENT TO CONSTANTLY XCHK THE VISUAL WITH THE LOC. SUPPLEMENTAL INFO FROM ACN 582886: OVERSHOT CTRLINE FOR RWY 17C AND LINED UP ON RWY 17R. GOT HIGH ON THE GS. BELOW 1000 FT AGL AFTER LNDG FLAPS HAD BEEN EXTENDED; I LOOKED AT THE LOC AND IT WAS OFF SCALE TO THE L. TURNED L TO LINE UP ON RWY 17C AND WAS ON EXTENDED CTRLINE FOR RWY 17C BY 500 FT AGL. DFW TWR NEVER SAID A WORD ABOUT OUR MISTAKE.