PBI Controller described a conflict event during the application of visual separation between an IFR and VFR traffic; the reporter noting positive altitude separation may have more appropriate until the aircraft passed.

Narrative

I was working all PBI airspace combined. I was handed off Air Carrier X from the Center descending VFR out of 13;500 FT. Aircraft Y was enroute to VRB at 7;000 FT. Air Carrier X was issued traffic on some VFR PA28's then on Aircraft Y. Aircraft Y was issued traffic on Air Carrier X; reported him in sight; and was told to maintain visual separation. Air Carrier X eventually got Aircraft Y in sight and; since he was VFR; he was not issued any control instructions. Air Carrier X continued descending directly at Aircraft Y and the pilot questioned Air Carrier X's intentions. Air Carrier X appeared to pass directly over Aircraft Y with 300 FT vertical separation. The pilot of Aircraft Y said his passengers were unhappy with the close proximity of Air Carrier X; but he never reported a near mid-air. Recommendation; since the closure rate of the aircraft was near 500 KTS I considered Air Carrier X's actions unsafe. Both aircraft did have one another insight; but Air Carrier X's continued descent did not seem appropriate for the aircraft. Both aircraft are larger turboprops and I don't know if they need to keep a minimum safe distance when VFR. I could have stopped Air Carrier X's descent above Aircraft Y; but they had one another insight and I didn't want to restrict an aircraft's ability to maintain VFR. If I knew of the aircraft's intentions to practically buzz the other I would have stopped his descent.