A 'TRIN' PLT; UPON ADVISING KIAD APCH HE WOULD CANCEL HIS IFR FLT PLAN UPON LNDG; AS PER THE NEW RESTRS; WAS ADVISED HIS IFR CLRNC WAS CANCELLED AND A FREQ CHANGE WAS APPROVED.

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IFR FLT PLAN FROM CHO TO JYO (LEESBURG EXECUTIVE ARPT); AN UNCONTROLLED AIRFIELD; WHICH IS LOCATED IN THE DULLES INT'L ARPT 'ENHANCED' CLASS B AIRSPACE. WX WAS CAVU. NOTAMS STATED NO INFLT CANCELLATION OF IFR IN ENHANCED B AIRSPACE EVEN IN VMC. POSSIBLE PROB; INFLT IFR CANCELLATION IN ENHANCED CLASS B AIRSPACE. I WAS CLRED BY DULLES APCH FOR A VISUAL APCH RWY 35 AT JYO ABOUT 15 NM WSW OF JYO. AT 3000 FT AND 6-8 NM FROM JYO; I CONTACT DULLES APCH STATING THAT I HAD VISUAL CONTACT WITH THE ARPT. I THEN STATED THAT I'D CONTACT DULLES APCH ON 118.55 (RCO) UPON LNDG TO CANCEL AND TO VERIFY THAT IFR CANCELLATION WAS REQUIRED ON THE GND (UPON LNDG). THE DULLES APCH CTLR IMMEDIATELY CAME BACK WITH AN IFR CANCELLATION AND FREQ CHANGE APPROVAL WHILE I WAS STILL AIRBORNE; ABOUT 6 NM NW OF JYO. WHAT WAS UNUSUAL; HOWEVER; IS THAT I WAS NOT ASKED TO CHANGE MY XPONDER TO VFR (1200). I MAINTAINED MY UNIQUE XPONDER CODE TO LNDG. AFTER LNDG; I CONTACTED THE LEESBURG FSS; LOCATED AT JYO; REGARDING THIS MATTER. THE FSS STATED; BASED ON RPTED TELEPHONE CONVERSATIONS BETWEEN JYO AND DULLES APCH; THAT DULLES APCH WAS NOW PERMITTING IFR CANCELLATIONS IN THE ENHANCED CLASS B PRIOR LNDG AT JYO DUE TO TFC CONGESTION PROBS. I WAS TOLD; 'NOT TO BE CONCERNED' ABOUT MY INFLT IFR CANCELLATION. THE FSS BRIEFER ALSO INDICATED THAT SOME VFR DEPS WERE BEING PERMITTED PRIOR TO PICKING UP AN IFR CLRNC. LATER THAT EVENING I WAS REVIEWING A LOCAL PLTS INTERNET LISTSERV AND DISCOVERED ANOTHER AREA PLT HAD THE SAME IDENTICAL PROB ON THE SAME DAY. APPARENTLY; HE CONTACTED DULLES APCH DIRECTLY VIA TELEPHONE THE NEXT MORNING AND THE SUPVR SAID 'UNDER NO CIRCUMSTANCES ARE INFLT IFR CANCELLATION PERMITTED.' PIC MAY HAVE ERRED BECAUSE HE ASSUMED THERE MAY HAVE BEEN RECENT CHANGES TO INFLT IFR CANCELLATION PROC (NOTAMS) WHILE HE WAS AIRBORNE ENROUTE TO DEST (IE DIDN'T HAVE LATEST NOTAMS IN THE COCKPIT). ATC PROBABLY ACTED TO CANCEL IFR ACFT INFLT BASED ON PRE-ENHANCED CLASS B OPS PROCS; TO EXPEDITE TFC AND THE FACT THAT THE WX WAS CAVU. CTLR CANCELLED IFR FLT INFLT; VIOLATION OF NOTAMS; PIC SHOULD HAVE INSISTED THAT FLT NOT BE CANCELLED UNTIL LNDG (ON GND); PIC LIABLE.