FORTIFIED COCKPIT DOOR ON MD80 OPENED ON TAKEOFF ROTATION; RESECURED BY #1 FLT ATTENDANT WHEN SAFE TO DO SO. FAA INSPECTOR IN COCKPIT DISCUSSED LOG BOOK ENTRIES WITH FLC.

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WHEN WE ARRIVED AT THE AIRPLANE AN FAA INSPECTOR MET US AND ADVISED US THAT HE WOULD BE PERFORMING A ROUTINE RTE INSPECTION. WE PUSHED FROM THE GATE AND STARTED THE ENGINES. I CALLED GND FOR TAXI INSTRUCTIONS AND WE BEGAN TO TAXI. THE TWR CTLR CLRED US INTO POSITION AND THEN FOR TAKE-OFF. THE ACCELERATION WAS NORMAL. ON ROTATION THE COCKPIT DOOR OPENED. AFTER WE REACHED A SAFE ALT THE #1 FLT ATTENDANT REACHED OVER AND CLOSED THE DOOR. DURING THE FLT THE FAA INSPECTOR QUESTIONED US ABOUT THE DOOR AND ASKED IF WE WERE GOING TO MAKE A LOGBOOK ENTRY ON ARR IN ST.LOUIS. IT WAS THEN THE CAPT ADVISED ME THAT THERE HAD BEEN A LOGBOOK ENTRY ON A PREVIOUS FLT. THE FACT THAT THE CAPT KNEW ABOUT THIS PREVIOUS LOGBOOK ENTRY AND DID NOT ASK ME TO REVIEW THE MINIMUM EQUIP LIST; LED ME TO BELIEVE THAT WE WERE IN COMPLIANCE WITH ALL REQUIREMENTS AND HAD COMPLIED WITH ALL PROCS. THERE WAS SOME DISCUSSION ABOUT THE NEW DOORS ON THE MD80 AND THE FACT THAT SOME ARE PRONE TO OPEN ON ROTATION AND OUR INSTRUCTIONS WERE TO MAKE A LOGBOOK ENTRY SO THEY COULD BE FIXED. AGAIN; THE FACT THAT THE NEW DOORS WERE PRONE TO OPEN AND THE CAPT DID NOT ASK TO REVIEW THE MEL FURTHER LED ME TO BELIEVE HE HAD ENCOUNTERED THIS ITEM IN THE RECENT PAST AND WAS FAMILIAR WITH THE PROCEDURE AND THE LIMITATIONS. THE FLT ATTENDANTS HAVE ALSO BEEN ADVISED TO VERIFY THE DOOR IS SECURE PRIOR TO TAKING THEIR SEATS FOR TAKE-OFF. AT NO TIME WAS THE DOOR LOCKING MECHANISM OR ANY PART OF THE DOOR NOT WORKING.