AN ACR MLG TURNED R TO AVOID A VFR ACFT.

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ARR INTO CVG ON BLUNT 8 -- APCHING BLUNT INTXN AT 12000 FT AND 250 KIAS. TCASII INDICATED AN ACFT APCHING MORE OR LESS HEAD ON AND CLBING THROUGH 11300 FT. WE ACQUIRED IT VISUALLY AND DETERMINED IT TO BE A LIGHT TWIN. BY THE TIME ATC CALLED OUT TFC TO US; WE HAD ALREADY STARTED A TURN TO THE R. TCASII INDICATED THE TWIN STILL CLBING THROUGH 11600 FT AT A DISTANCE OF LESS THAN 1 MI. ATC'S TFC CALL REFERRED TO THE TWIN AS CLBING TO 11500 FT. THIS INDICATES TO ME A PLANNED PROX OF 500 FT SEPARATION ON AN ARR INTO A VERY BUSY ARPT. TCASII WAS INDICATING A CLB TO AVOID THE CONFLICT AND; UNDER IMC; WOULD HAVE DICTATED THAT ACTION. ANY DEP FROM ALT WHICH IS INITIATED AS AN EVASIVE MANEUVER WOULD LOGICALLY INVOLVE AT LEAST 3-500 FT WHICH APPEARS TO BE THE PLANNED MARGIN OF SEPARATION INTO CVG. THE TWIN WOULD HAVE PASSED DIRECTLY UNDER US IF WE HAD NOT TURNED AND IF IT HAD MAINTAINED 11500 FT. THIS MEANS THAT NO HORIZ SEPARATION WAS PLANNED AT ALL. SCATTERED CLOUDS IN THE AREA PERMITTED VFR OPS BUT COULD HAVE ALSO CONTRIBUTED TO NO VISUAL CONTACT. AN EARLIER ATC CALL ON THE TFC WOULD NOT HAVE HELPED US FIND THE TWIN VISUALLY ANY QUICKER.