A B737 FLT CREW FOUND THEY WERE NOT NOTIFIED OF A MAINT DISCREPANCY PRIOR TO FLT.

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AFTER AN ACFT CHANGE; WE DEPARTED FOR AN UNEVENTFUL FLT TO PHL. WE ARRIVED EARLY; GIVING US ABOUT 45 MINS ON THE GND. ABOUT 15 MINS PRIOR TO DEP; I STEPPED INTO THE COCKPIT AND NOTICED THE INBOUND DR STILL ON THE PEDESTAL. AS I GLANCED AT IT; I NOTICED A NOTATION UNDER THE HDG 'FLT REMARKS.' IT STATED 'LOAD AGENT ASSISTING FLT NOTICED A WEAKNESS IN REAR BIN FLOOR. MIGHT BECOME A CRACK. PLEASE CHK.' TO SAY I WAS SURPRISED IS AN UNDERSTATEMENT. NO ONE HAD NOTIFIED ME OR MY FO ABOUT THE PROB EITHER BEFORE DEPARTING OR ON ARR IN PHL. I ASKED THE OPS AGENT WHAT HE KNEW ABOUT IT. HE SAID THAT HE ASSUMED THAT I HAD KNOWN ABOUT IT AND HAD TAKEN CARE OF IT. (MAINT HAD PREVIOUSLY BEEN TO THE ACFT FOR AN UNRELATED PROB.) I ASKED THAT THE MECH RETURN TO THE ACFT TO INSPECT THE AFT BIN. DURING HIS INSPECTION HE DISCOVERED A 15 INCH LONG CRACK IN THE FLOOR OF THE BIN RUNNING ADJACENT TO A STRINGER. I CONTACTED DISPATCH AND MAINT TO RELAY THE STORY AND TO EXPRESS MY CONCERN THAT THE ACFT WAS ALLOWED TO DEPART WITH NO ATTEMPT TO NOTIFY THE CREW. THEY AGREED WITH MY CONCERNS AND STATED THEY WOULD BEGIN A REVIEW TO DISCOVER WHERE THE BALL GOT DROPPED. WHILE THIS TURNED OUT TO NOT BE A STRUCTURAL PROB; IT CERTAINLY COMPROMISED THE FIRE CONTAINMENT PROPERTIES OF THE CARGO BIN. THEREFORE; THE ACFT WAS DISPATCHED UNDER THE PROVISIONS OF MEL 25-16.