AUS Approach Controller described technical loss of separation when they failed to issue visual approach wake turbulence advisory after trailing air carrier reported heavy traffic in sight and following.

Narrative

I was working final approach in a moderately busy session. Runway 17R closed to arrivals; putting all aircraft in queue for landing Runway 17L. Air Carrier X; a heavy jet; was cleared for a visual approach to Runway 17L; and switched to Tower frequency. Air Carrier Y; a CRJ7; was issued traffic on the heavy jet and CRJ7 reported the Heavy Jet and the airport in sight. I acknowledged the transmission and instructed Air Carrier Y to maintain 3;000 FT; and issued traffic on a primary target ahead. I never went back and instructed Air Carrier Y to follow the traffic; and did not issue a wake turbulence advisory. I shipped Air Carrier Y off to the Tower with no clearance for the approach; thinking I had done so. A Supervisor approached me and told me that the CRJ7 had an overtake on the Heavy Jet; and asked me if the aircraft was following the Heavy visually. Believing that to be the case; I said yes. As I was working; I tried to replay the scenario in my head; and could not remember issuing a wake turbulence advisory to Air Carrier Y; so I called the Tower and asked the Controller to issue the advisory for me; which he did. Air Carrier Y had already initiated descent on his own; and was then cleared to land by the Tower. After getting relieved from the position; I had time to think about the situation; and could not replay in my head issuing an approach clearance; so I asked to listen to the voice recording. Sure enough; Air Carrier Y reported the Heavy Jet in sight; but I did not issue the clearance. The radar showed it was 4.69 miles in trail; of course I needed 5 miles. Altitude was maintained all the way down; although that does not matter in this situation; closest point was 1100 FT vertical when the Heavy touched down. In hindsight; when the Supervisor approached me I should have acknowledged even the slight possibility that all was not as it should have been and called the Tower to reissue the clearance. Since Air Carrier Y was already following the Heavy anyway; just the matter of a few words would have salvaged the situation.