ADS Controller described potential conflict event involving an IFR over flight and a VFR departure aircraft; reporter indicted they should have issued traffic to the departure prior to frequency change.

Narrative

It was very slow and approach called me on the shout line and requested a descent for Aircraft Y going into DAL. Normally they are capped at 3;000; but since I didn't have any departures I gave control. Aircraft Y was about 7 miles away when this happened. Aircraft X calls me later ready for VFR flight following and I cleared him and assigned maintain VFR at or below 2;000. They take off and I switched them to Departure. I then noticed Aircraft Y at 020 and Aircraft X was at 015; the were both now talking to Departure. Out the window it looked find and Aircraft Y passed behind the VFR Aircraft X and there was 400-500 FT between them vertically. If they had both been IFR this would have been an error. The story on the scope looked worse than outside since the CA (conflict alert) was going off. It is the pilots responsibility to See and Avoid when IFR but I wish I had told him about the traffic before hand even though I thought the crossing traffic would; and did; pass behind him. Recommendation; I will force myself to stay 100% focused even when it is slow.