B757-200 FLT CREW OBSERVES THAT EHSI DATA DOES NOT CORRESPOND TO ILS AND VISUAL INDICATIONS ON APCH TO PHX. EXCESSIVE IRS DRIFT RATES NOTED AFTER LNDG.

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ALL PREFLT; DEP; CRUISE; AND DSCNT ACTIVITIES WERE NORMAL. OTHER THAN BEING REROUTED OVER DRK AND THE COYOTE ARR DUE TO WX; THE FLT WAS COMPLETELY ROUTINE UNTIL THE LAST 5 MINS. WHILE ON ABOUT A 20 NM BASE LEG FROM THE N FOR THE VISUAL TO RWY 8; THE ILS FOR RWY 8 WAS TUNED AND INTERCEPT LEG TO WAZUP WAS EXECUTED IN THE FMC. ABOUT 10 MILES OUT; WE WERE GIVEN A NORMAL DOGLEG/INTERCEPT HDG AND CLRED THE VISUAL APCH TO RWY 8. WHEN ESTABLISHED ON THE HDG; I NOTICED BOTH EHSI'S SHOWED US TRACKING DIRECTLY TO THE ARPT. WHILE I CONTINUED TO FLY; FO DOUBLE-CHKED ALL SETTINGS AND CONFIRMED WE SELECTED THE CORRECT ARPT; THE CORRECT RWY; AND THE CORRECT ILS FREQUENCY/COURSE. BOTH NAV RADIOS WERE IN AUTO THROUGHOUT THE FLT EXCEPT FOR PRE-TUNING DEP AND ARR VORTACS IN CASE THEY WERE NECESSARY. NO FAILURES OF ANY KIND WERE ANNUNCIATED; THE AIRPLANE WAS LOST AND IT DIDN'T KNOW IT. I FLEW VISUALLY AND THE ILS INDICATIONS WERE ALL NORMAL. THE EHSI'S CONTINUED TO SHOW US OFF COURSE; WITH THE FINAL APCH COURSE AND ARPT SEVERAL MILES TO THE S OF OUR POS. WE TAXIED INTO GATE; AND LEFT THE ILS TUNED AND THE IRS'S IN NAV. WHEN PHX WAS ENTERED IN THE FIX PAGE; IT SAID THE ARPT WAS 5 MILES TO THE S OF OUR CURRENT POS AT THE GATE. THIS ACFT WAS TOTALLY LOST. IRS DRIFT RATES WERE ALL ABOUT 4.5 NM/HR. IF THIS HAPPENED WHILE IN THE WX AND A NON RADAR ENVIRONMENT (SUCH AS MEXICO); THE GPWS ACTIVATION MAY HAVE BEEN NECESSARY TO KEEP THE ACFT FROM RUNNING INTO TERRAIN WHILE SO FAR OFF COURSE. BUT EVEN IF IFR IN A RADAR ENVIRONMENT; THIS FAILURE COULD EASILY HAVE RESULTED IN A SERIOUS SEPARATION CONFLICT; ESPECIALLY AT AN ARPT WITH SIMULTANEOUS PARALLEL APCHS. I FULLY BRIEFED THE ONCOMING CAPT AND CONTACTED THE 757 FLEET MGR IMMEDIATELY FROM THE GATE PODIUM; THEN WENT TO MY NEXT FLT. THIS ANOMALY; SIMILAR TO WHAT I EXPERIENCED MANY YRS AGO IN A 737 EFIS ACFT; REQUIRES IMMEDIATE ACTION AND THE INVOLVEMENT OF BOEING. WHEN AN ACFT IS OPERATED UNDER ALL THE STANDARD OPERATING PROCS AND GETS SO FAR OFF COURSE WHILE CLOSE TO THE GND; WITHOUT ANY INDICATIONS OF A FAILURE; IT IS A SERIOUS SAFETY HAZARD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE INITIAL POSITION LOADED IN THE FMC WAS CORRECT AND ALL IRS INDICATIONS WERE NORMAL UNTIL THE APCH AT DEST. RPTR STATED THAT THERE WAS NO 'IRS NAV ONLY' WARNING AT ANY TIME. RPTR STATED THAT HE MADE A LOGBOOK ENTRY ON THE FMC AND THAT THE MAINT TECHNICIAN'S CORRECTIVE ACTION WAS TO SHUT DOWN THE IRS AND REALIGN THE IRU'S. RPTR STATED THAT THE ACFT DEPARTED ON THE NEXT SCHEDULED FLT. RPTR STATED THAT HE HAS HEARD OF OTHER FLT CREWS EXPERIENCING THIS PROB ON THE B757; BUT THAT HE HAD NOT SEEN THE PROB BEFORE EXCEPT ON A B737. RPTR SAID THAT HE FEELS THAT THE FMC WAS NOT REFINING ITS POS FROM THE VOR/DME DATA. RPTR STATED THAT HE HAD HEARD THAT THERE HAD BEEN A RECENT FMC SOFTWARE UPDATE; BUT DID NOT KNOW IF HIS COMPANY HAD INSTALLED IT.