EXTRA 300L AEROBATIC ACFT PLT AND AN ACR COMMUTER ACFT HAD AN NMAC AT BED.

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CONDITIONS WERE UNLIMITED CEILING AND VISIBILITY. I WAS RETURNING TO BED AFTER AN AEROBATICS PRACTICE SESSION. I CALLED THE TWR PASSING MINUTEMAN ARPT (6B6) INFORMING THEM OF MY POS; AND THAT I HAD THE INFO FROM ATIS. I WAS TOLD TO RPT MIDFIELD L DOWNWIND; WHICH I ACKNOWLEDGED. WHILE ON THE 45 DEG ENTRY TO DOWNWIND (RWY 29); I NOTICED A COMMUTER FLT DEPARTING ON RWY AND CLBING STRAIGHT OUT. I THEN DIVERTED MY ATTN TO OTHER POSSIBLE TFC ON XWIND OR DOWNWIND. MY MAIN CONCERN WAS TRAINING ACFT PRACTICING TOUCH-AND-GOES. THESE ACFT ARE USUALLY HARD TO SPOT AND MUCH SLOWER THAN MY ACFT. WHILE MY ATTN WAS FOCUSED ON SPOTTING THIS TFC; I NOTICED SOME MOTION TO MY L WITH MY PERIPHERAL VISION. I TURNED MY HEAD L AND SAW THE COMMUTER ACFT IN A CLB COMING STRAIGHT TOWARD ME. APPARENTLY; HE HAD EXECUTED A L TURN WHILE MY ATTN WAS DIVERTED AWAY FROM HIM. BEING STRAPPED INTO MY ACFT WITH A 5-POINT AEROBATIC HARNESS; I IMMEDIATELY EXECUTED A DIVE AND PASSED ABOUT 200 FT UNDER THE COMMUTER. BOTH G METERS IN MY ACFT REGISTERED NEGATIVE 4.5 G'S. IF I HAD NOT BEEN IN A VERY RESPONSIVE AEROBATIC ACFT; I WOULD HAVE COME MUCH CLOSER BUT PROBABLY WOULD STILL HAVE MISSED THE COMMUTER PLANE BY ABOUT 50 FT. SKY CONDITIONS WERE PERFECT AND THE EXTRA HAS EXCELLENT VISIBILITY; YET WE STILL DIDN'T SEE EACH OTHER UNTIL THE LAST MIN. THE TWR HAS RADAR CAPABILITY; THE XPONDER WAS ON; BUT NEITHER ACFT RECEIVED AN ADVISORY. THIS IS VERY UNUSUAL FOR INBOUND/OUTBOUND ACFT AT BED.