FLT CREW OF A320; CONCERNED ABOUT FUEL STATE AND POSSIBLE DIVERSION; FAILED TO EXTEND GEAR UNTIL LATE IN APCH WHEN ACFT'S MASTER WARNING SYS ALERTED THEM.

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WE WERE INBOUND TO MEX. WE WERE ISSUED 25 MINS OF HOLDING OVER OTU VOR. WE DECIDED TO DIVERT; BUT JUST AS WE WERE ABOUT TO START OUR DIVERSION; MEX APCH SAID THEY COULD CLR US FOR AN APCH. WE WERE VECTORED FOR THE ILS RWY 23L MEX. THE TSTM THAT WAS OVERHEAD HAD MOVED S AND W OF THE FIELD. WE HAD DECLARED MINIMUM FUEL WITH THE APCH CTLR. WE WERE PLANNING ON LNDG AT MEX WITH 6700 LBS OF FUEL AND ACA (OUR ALTERNATE) WITH 3700 LBS FUEL. WE ACQUIRED THE LNDG RWY VISUALLY AT 6.5 MI FROM MEX VOR. WE WERE ISSUED A SPD OF 160 KTS. THE CAPT ASKED FOR FLAPS 3 DEGS. I QUERIED THE APCH CTLR IF WE SHOULD SWITCH TO TWR; HE SAID AFFIRMATIVE. I RPTED TO MEX TWR; THEY TOLD US TO SLOW TO FINAL APCH SPD. THE CAPT SELECTED MANAGED SPD AND FLAPS FULL. I TOOK THE CHKLIST OUT OF THE HOLDER; TO COMPLETE THE FINAL DSCNT CHK. AT THAT POINT THE TWR ASKED IF WE COULD SIDESTEP TO RWY 23R; BECAUSE OF TFC PRECEDING US. I PUT THE CHKLIST BACK IN THE HOLDER WITHOUT ACCOMPLISHING THE FINAL DSCNT CHK. I CHANGED THE ACTIVE RWY IN THE FMGC TO RWY 23R JUST FOR DSCNT GUIDANCE. I MISSED THE 1000 FT CALLOUT; AND AT 800 FT; WE RECEIVED THE MASTER-WARNING FOR THE GEAR NOT-DOWN. WE DECIDED TO LOWER THE GEAR AND COMPLETE THE APCH. I COMPLETED THE FINAL DSCNT CHKLIST; THE LATE 1000 FT CALL; AND A STANDARD (ON ALT) 500 FT CALL. BY 500 FT; WE WERE STABILIZED WITH ALL SOPS COMPLIED WITH. FACTORS INCLUDED OUR FUEL STATE; CHANGE OF RWYS; CHANGING THE FMGC TO MATCH THE RWY (FOR VERT GUIDANCE); WX IN AND AROUND MEXICO CITY ARPT; AFTER THE SIDESTEP; WE WERE BELOW THE RWY 23L VASI GS WHICH CAPTURED SOME MORE OF MY ATTN OUTSIDE THE AIRPLANE; THE PNF (ME) NOT MONITORING THE STATE OF THE AIRPLANE (LNDG GEAR-DOWN) MORE CLOSELY; TAKING THE CHKLIST OUT OF ITS HOLDER AND NOT COMPLETING THE CHKLIST IMMEDIATELY. SUPPLEMENTAL INFO FROM ACN 587104: AT ABOUT 4 MI OUT; WE WERE ASKED TO SIDESTEP TO RWY 23R. I DISENGAGED THE AUTOPLT AND MANEUVERED TO THE R TO LINE-UP FOR RWY 23R (WE ALSO LEVELED OFF TO REGAIN PROPER GS ALIGNMENT FOR THE NEW RWY). DURING THIS MANEUVER; I SPECIFICALLY RECALL THE FO REMOVING THE CHKLIST FROM THE STOWAGE POUCH. WE WERE HOWEVER QUITE BUSY AT THIS TIME; STILL LOOKING FOR THE DC9 TFC AND COMMUNICATING WITH TWR. UNFORTUNATELY THE CHKLIST WAS RETURNED TO THE POUCH WITHOUT EVER HAVING BEEN READ. REALIZING MY MAJOR OVERSIGHT; WE IMMEDIATELY LOWERED THE GEAR AND CONTINUED TO LAND ON RWY 23R. I REALIZE THAT THIS MANEUVER IS AGAINST COMPANY POLICY AND THAT A GAR WOULD HAVE BEEN IN ACCORDANCE WITH OUR SOPS. I CHOSE TO PROCEED TO AN UNEVENTFUL LNDG FOR THE FOLLOWING REASONS. OUR GEAR WAS EVENTUALLY DOWN AND LOCKED BY THE 500 FT AGL CALLOUT POINT AND I WAS OVERLY CONCERNED ABOUT OUR MINIMUM FUEL SIT. THE GROSS OVERSIGHT ON MY PART HAS LEFT AN INDELIBLE IMPRESSION ON ME.