POTENTIAL CONFLICT ENSUES BTWN ACR 757 AND COMMUTER METRO MERLIN.

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WE WERE FLYING A SCHEDULE FLT FROM SLC TO IAD. WE WERE IN THE FINAL PHASE OF FLT FLYING THE JASEN ONE ARR TO IAD. ZDC CLRED US TO THE DRUZZ INTXN AND TO CROSS DRUZZ AT 11000 FT AND 250 KTS. WE WERE APPROX 2 NM W OF DRUZZ AT 11000 FT AND 250 KTS WHEN ZDC HANDED US OFF TO DULLES APCH. I WAS FLYING THE ACFT; AUTOPLT ON; LNAV ACTIVE; ALT HOLD (ALT HOLD) AND AUTOTHROTTLES ON IN SPD MODE. THE FO WAS CHANGING FREQS ABOUT TO CHK IN WITH APCH CTL. AT THAT TIME WE RECEIVED A 'TFC; TFC' FROM THE TCASII. TCASII SHOWED AN ACFT AT APPROX 11.5 O'CLOCK; APPROX 800 FT BELOW US. SOON THEREAFTER TCASII SYS GAVE US A 'CLB NOW' RA. I INITIATED A CLB TO THE GREEN ARC; DISCONNECTED THE AUTOPLT AND APPLIED PWR. AS I CLBED TCASII MESSAGE WAS 'MONITOR VERT SPD.' THE ACFT PASSED OFF MY L WING AND I REMEMBER SEEING A RED MINUS 3 ON MY TCASII VSI. I MADE VISUAL CONTACT WITH THE ACFT; IT BEING A MERLIN TYPE ACFT WITH POWDER BLUE STRIPES. WE WERE THEN ABLE TO TALK WITH APCH CTL AND ADVISED THEM OF THE RA. THEY APOLOGIZED AND TOLD US THAT THE ACFT WAS OUT OF THEIR AIRSPACE. THE PROB SEEMED TO BE THAT APCH HAD THOUGHT THAT THE CTR HAD ADVISED US OF THE ACFT; VICE VERSA; AND THE FACT THAT WE WERE IN THE MIDDLE OF A HAND OFF AND NO ONE HAD POSITIVE CTL. THE FACT THAT A MERLIN TYPE ACFT WAS ON A VFR CLB ON A HAZY DAY WITH TSTMS IN THE AREA COMPOUNDED THE SIT. ALSO INVOLVED WAS THE FACT THAT HE WAS CLBING VFR NEAR AN APCH CTL 'GATE' TO A MAJOR ARPT. WE WERE FLYING A B-757 ACFT.