AN L1011 BEGAN ROLLING BACKWARDS AT THE GATE AFTER CHOCKS WERE IN AND PARKING BRAKE TAKEN OFF AT ATL; GA.

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AFTER ENG SHUTDOWN AT GATE; THE RAMP AGENT GAVE THE 'CHOCKS IN' SIGNAL AND THE PARKING BRAKE WAS RELEASED. APPROX 5 SECONDS LATER; THE ACFT BEGAN SLIDING BACKWARDS; PUSHING BACK THE CHOCKS. THE ACFT SLID BACK 1-2 FT BEFORE THE BRAKES COULD BE REAPPLIED. NO DAMAGE TO THE ACFT OR GND EQUIP WAS RPTED AND THERE WAS NO RPT OF GND PERSONNEL; CREW OR PAX INJURY. THIS APPEARS TO BE THE RESULT OF A CHAIN OF EVENTS AS FOLLOWS. WE DEPARTED BOS ON FLT THAT MORNING. SINCE IT WAS SNOWING; THE ACFT WAS DEICED WITH 100% TYPE I FLUID. WE ARRIVED IN ATL AND PARKED AT GATE. WE WERE SCHEDULED TO FLY THE SAME ACFT TO TPA AND BACK. DURING PREFLT; THE SO NOTICED FLUID ON THE BOTTOM OF THE WINGS AND PUDDLES UNDER THE WHEEL WELLS. WE CALLED MAINT. THE MECH INSPECTED THE ACFT AND RPTED THAT THE FLUID WAS RESIDUAL DEICING FLUID. HE SAID THAT DEICING FLUID WAS RECOGNIZABLE BECAUSE OF ITS SLIPPERY APPEARANCE AND FEEL. WE THEN FLEW FLT TO TPA AND FLT BACK TO ATL WITHOUT INCIDENT. WHEN WE ARRIVED IN ATL IT WAS RAINING AND WE PARKED AT GATE AGAIN; WHERE THE ACFT SLID BACKWARDS EVEN THOUGH CHOCKS WERE IN PLACE. IT APPEARS THAT THE PUDDLES OF DEICING FLUID; COMBINED WITH THE RAIN; MADE THE AREA AROUND THE ACFT SO SLIPPERY THAT THE CHOCKS WERE UNABLE TO HOLD THE ACFT WHEN THE PARKING BRAKE WAS RELEASED. I DO NOT KNOW WHETHER THE SLOPE OF THE RAMP AROUND GATE IS GREATER THAN NORMAL. THE WIND WAS NOT A FACTOR. WE NOTIFIED THE RAMP TWR BY RADIO AND I CALLED THE COORDINATOR ON THE PHONE TO EXPLAIN THE SIT AND TO INSURE THAT SOMEONE WOULD INSPECT THE RAMP AREA AROUND GATE.