ACR READS BACK AND FLIES WRONG CLRNC AND CTLR DOES NOT CORRECT IT UNTIL IT IS APPARENT IN THE AIR.

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THE CAPT AND I WERE OPERATING A SCHEDULED FLT FROM ORD TO PHL. WE HAD A NORMAL PUSHBACK AND TAXI FOR TKOF. THERE WAS NO RUSH TO COMPLETE CHKLISTS. WHEN OUR TURN CAME FOR TKOF; WE WERE CLRED BY ORD TWR TO 'POS AND HOLD RWY 22L.' WE WERE THEN GIVEN THE CLRNC: 'TURN L HDG 090 DEGS; RWY 22L CLRED FOR TKOF.' THE CAPT MADE A NORMAL TKOF AND COMPLIED WITH THE L TURN TO A 090 DEG HDG. AFTER ROLLING OUT OF THE TURN; ORD TWR ASKED US WHAT OUR HEADING WAS. I TOLD HIM 'HDG 090 DEGS.' HE IMMEDIATELY ISSUED US A CLRNC TO TURN R TO A 180 DEG HDG; ADDING THAT THE 090 DEG HDG HE ISSUED WAS NOT FOR US; BUT THE ACFT THAT DEPARTED PRIOR TO US. IT WAS OBVIOUS FROM HIS EXCITED TONE OF VOICE THAT HE WAS CONCERNED ABOUT A LOSS OF SEPARATION WITH ANOTHER ACFT. WE IMMEDIATELY COMPLIED WITH THE CLRNC TO TURN R TO HDG 180 DEGS. WE DID NOT RECEIVE ANY TFC OR RA'S FROM OUR TCASII SYS; AND THE TWR CTLR MADE NO MENTION TO US THAT THERE MIGHT HAVE BEEN A LOSS OF SEPARATION. AFTER ANOTHER HEADING CHANGE WE WERE HANDED OFF TO ORD DEP; AND THE FLT CONTINUED NORMALLY. BOTH THE CAPT AND I HAVE NO DOUBT IN OUR MINDS THAT THE ORD TWR CTLR CLRED US TO TURN L TO A 090 DEG HDG WHEN HE CLRED US FOR TKOF. THE TWR CTLR OBVIOUSLY THOUGHT OTHERWISE. AS A PROFESSIONAL PLT; I CAN TELL YOU THAT SUCH OCCURRENCES HAPPEN EVERYDAY AT MAJOR ARPTS. THE WORKLOAD FACED BY CTLRS AND PLTS EVEN DURING 'NORMAL' OPS CAN BE QUITE HVY; THUS INCREASING THE CHANCE FOR MISTAKES. AS CAPACITY ENHANCEMENT TOOLS ARE IMPLEMENTED (LAND AND HOLD SHORT OPS; PRM APCHS; ETC); WORKLOAD AND ERRORS WILL RISE. ALSO DISTURBING IS THE FACT THAT THE FAA HAS RECENTLY ABSOLVED CTLRS FROM THE REQUIREMENT TO CORRECT INCORRECT PLT READBACKS. THIS POLICY IS CLRLY A DETRIMENT TO SAFETY AND IS THE PRIMARY REASON I AM FILING THIS NASA RPT. IT IS GERMANE TO THE SIT DESCRIBED ABOVE. IF OUR READBACK WAS INCORRECT; THE TWR CTLR SHOULD HAVE CORRECTED IT.