AS B737-300 DESCENDED THROUGH 500 FT ON FINAL APCH; 'TOO LOW; FLAPS' WARNING ACTIVATED. FLT CREW SELECTED LNDG FLAPS AND CONTINUED.

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DURING R BASE LEG FOR A VISUAL APCH MCO RWY 35R; ATC SWITCHED US TO RWY 35L WITH OUR CONCURRENCE. BOTH PLTS SWITCHED TO RWY 35L ILS FREQ AS BACKUP TO VISUAL. LOC SIGNAL WAS OBTAINED; BUT NO GS SIGNAL. WE HAD ALREADY STARTED CONFIGN SEQUENCE AND WERE CONFIGURED WITH FLAPS 15 DEGS AND LNDG GEAR DOWN ON A NORMAL DSCNT PATH. AS PLT MONITORING; I WAS ATTEMPTING TO DISCERN WHY GS WAS UNAVAILABLE FOR PF. THE 1000 FT CALL WAS MADE WITH AIRSPD OF 150 KTS. V-TARGET WAS 141. SHORTLY THEREAFTER; AT 500 FT AGL; WE RECEIVED THE WARNING 'TOO LOW; FLAPS.' PF CALLED FOR FLAPS 30 DEGS AND BEFORE LNDG CHKLIST. I COMPLIED WHEN I SHOULD HAVE DIRECTED A GAR. LNDG WAS COMPLETED WITHOUT INCIDENT; AND TAXI TO GATE WAS UNEVENTFUL. IN AN ATTEMPT TO OFFER SUPPORT TO THE PF; I HAD ALLOWED MYSELF TO BECOME DISTR DURING A CRITICAL PHASE OF FLT WITH AN UNNECESSARY PIECE OF APCH GUIDANCE FOR THE TYPE OF APCH FLOWN. PROPER PRIORITIZATION AND APPLICATION OF PLT MONITORING DUTIES WOULD CERTAINLY HAVE PREVENTED THIS. PLT MONITORING IS ALSO FLYING; NOT NECESSARILY 'HANDS ON.' WHETHER PF OR PLT MONITORING; AVIATE FIRST AND AVOID/CONTAIN UNNECESSARY DISTRS THROUGH CORRECT IDENT AND PRIORITIZATION OF PERCEIVED PROBS.