A B757 CREW DESCRIBES A TAIL STRIKE CAUSED BY AN AFT CARGO BAY BAGGAGE SHIFT THAT MOVED THE CG AFT AT ROTATION.

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OUR FLT OUT OF ORD WAS DELAYED UNTIL XA00 DUE TO INBOUND ACFT. WX WAS 'MILE WITH' BLSN WINDS OUT OF THE SW. WE DE-ICED AT THE GATE AND TAXIED TO RWY 32R FULL LENGTH. THE ONLY THING THAT WAS DIFFERENT WAS DELAYING THE CONFIGN DUE TO POSSIBLE ICE ON THE TXWYS. I DIDN'T FEEL RUSHED IN ANY WAY. TKOF WAS NORMAL TO THE POINT OF ROTATION. AT VR THE ACFT APPEARED TO START THE ROTATION NORMALLY BUT SHORTLY AFTER NOSEWHEEL LIFT-OFF THE ROTATION RATE INCREASED. BOTH PLTS FELT WHAT SEEMED LIKE A THUMP ASSOCIATED WITH CARGO SHIFTING. MY FIRST REACTION WAS TO CHK THE ENGS (OPERATING NORMALLY) AND THEN I LOOKED AT THE TRIM AND YOKE POS (BOTH APPEARED NORMAL). WE COMPLETED THE AFTER TKOF CHKLIST AFTER ANALYZING THE ACFT AND TALKING TO THE FLT ATTENDANT. FLT ATTENDANT INDICATED TO THE CAPT THAT THEY FELT A LOUD THUMP IN THE FLOOR OF THE AFT CABIN. NORMAL CLBOUT FOLLOWED. I CONTINUED TO CHK THE ENGS AND PRESSURIZATION WHILE WE DISCUSSED THE 'ANOMALY.' WE DISCUSSED CARGO SHIFT AND TAIL STRIKE. THE CAPT INDICATED THE ACFT REALLY WANTED TO PITCH UP DURING ROTATION AT ABOUT THE TIME WE HEARD THE THUMP AND HE THOUGHT CARGO SHIFTED. HE DID NOT THINK THAT HE HAD A TAIL STRIKE. WE CONTINUED ON THE PRESUMPTION OF A CARGO SHIFT AND MONITORED THE PRESSURIZATION AND TRIM SETTINGS. THE LNDG WAS UNEVENTFUL. UPON PARKING WE CHKED THE TAIL AND DISCOVERED A LONG SKID AND POSSIBLE CRACK. BAGGAGE HANDLERS NOTED THAT ALL OF THE BAGS WERE AFT BY THE DOOR AND THE RESTRAINT WAS BROKEN. MAINT WAS NOTIFIED. THE CAPT THEN CALLED FLT DUTY MGR. OF NOTE: 1) NEITHER CAPT OR FO HAVE EVER EXPERIENCED A TAIL STRIKE FOR COMPARISON. 2) THE FLT ATTENDANT DIDN'T GIVE US ACCURATE INFO UNTIL AFTER THE FLT HAD LANDED. THEY HAD ASSUMED THAT SINCE I SAID THE ACFT WAS OK UP FRONT THAT EVERYTHING WAS OK. I DEBRIEFED THEM ON NEEDING ALL OF THE INFO IN A TIMELY MANNER. ONCE ON THE GND I WAS TOLD THAT IT WASN'T JUST A THUMP IN THE FLOOR; IT WAS THE WHOLE TAIL SHAKING. 3) I; THE PNF COULD NOT MAKE AN ASSESSMENT AS TO THE FEEL OF THE ACFT AT ROTATION OR THE EXACT POINT OF THE THUMP.