NMAC BTWN AN SMA SINGLE ENG LAND AND AN ULTRALIGHT ACFT IN THE ARPT TFC PATTERN.

Narrative

I CONDUCTED A VFR FLT FROM BFI TO ARLINGTON ARPT. LOW CLOUD MARINE FOG; VARIABLE 1200-1500 AGL; VISIBILITIES 7 MI; BREAKS IN OVCST. NOTAMS FOR ARLINGTON CITE TXWY AND RAMP CONSTRUCTION; SHARP BUMPS IN PAVEMENT; ETC. I UTILIZED LATEST ALTIMETER SETTING AS RPTED BY PAINE TWR (EVERETT; WA) APPROX 12 MI S OF ARLINGTON (I BELIEVE IT WAS 30.07). AT ANY RATE; UNICOM 122.7 ADVISED TFC LNDG TO S RWY 16. I RPTED 45 TO DOWNWIND. SAW NO TFC; HEARD AN ACFT RPTING TKOF STAYING IN THE PATTERN. I RPTED DOWNWIND ESTABLISHED AT 900 FT AGL FOR CLOUD CLRNC. SAW NO TFC. PROCEEDED BASE LEG. RPTED TO UNICOM FINAL; LNDG RWY 16 RPTED; SAW NO TFC. LANDED AND TAXIED TO RAMP. AT THIS TIME; A CAR PULLED UP AND A GENTLEMAN WALKS UP AND ASKED WERE YOU FLYING THIS PLANE WHICH I REPLIED IN THE AFFIRMATIVE. HE ASKED; 'DO YOU KNOW WHAT YOU DID WRONG?' I REPLIED I WAS NOT AWARE OF DOING ANYTHING WRONG; WHAT'S UP? IT SEEMS THAT AN ULTRALIGHT ACFT CONVENTION WAS BEING HELD AT THE W SIDE OF THE FIELD WHERE ULTRALIGHTS ARE HANGARED AND THIS FELLOW CLAIMS A NEAR MISS OCCURRED DURING MY DOWNWIND LEG. THAT I SHOULD HAVE BEEN ON A DOWNWIND LEG FARTHER W OF THE FIELD (R-HAND TURNS LNDG S) APPROX 2 MI!