TPA TRACON Controller described proximity event at 10;000 FT; relying on automation assist tool to determine separation; resulting in minimum spacing event.

Narrative

I was descending the southbound A319 to 6;000 FT into Tampa and instructed him to turn left 15 degrees for traffic that would cross out in front at 10;000 FT; northwestbound. I turned on my 'MIN' key to verify that my turn would be enough to sustain horizontal separation. I witnessed the 'MIN' alert go from approximately 1.59 miles to greater than 3 miles after the A319 completed his turn. After performing some other controller task; I went back to the A319 and witnessed the 'MIN' alert go down to 3.02 miles in separation. I then looked down to turn off the 'MIN' key and as I looked back up I noticed that the separation went down to 2.99 miles at the same time I hit the enter button. I went to call traffic and inadvertently knocked my headset off my ear. After which the aircraft passed each other without incident. I then turned the A319 back toward Tampa and proceeded the descent to a further altitude. Neither aircraft were made aware of each other. And neither aircraft were in a position of compromise. I use the 'MIN' key a lot and have never noticed separation decrease after all turns were completed. I never turned the A319 or the PA46 after my initial turn. It is possible that either aircraft deviated slightly for a cloud without my knowledge. I acknowledge that I should have called traffic earlier; which may have averted any concern should they had been in visual contact with each other.