PCT Controller described a potential conflict when IAD Tower assigned a departure heading that conflicted with with a HEF arrival after coordination had been completed; the reporter suggesting procedural changes.

Narrative

I was working ASPER and TILLY; the north and south departure positions for IAD; combined shortly after the conclusion of an arrival bank. Traffic was very light. An SF340; was being vectored for the ILS 16L approach at HEF by the BARIN; IAD's South Arrival Controller. He initiated an automated point out to me which I accepted. He then initiated an automated point out to Local 3 at IAD Tower; handling the west side of the airport; who also accepted. When the SF340 was approximately 5 miles west of the 16L localizer travelling east bound at 2;600 FT; a B737 departed IAD Runway 30 climbing to 3;000. When he checked in I asked for his heading and he replied that he had been assigned a 250 heading by the Tower and was beginning his turn. This heading placed him on a direct intercept course with the SF340; so I instructed him to turn back to a heading of 300 and issued a climb to 5;000 FT; along with calling traffic on the SF340. The B737 read back the instructions and made the turn in time to ensure separation; though I did not verify this; it appeared to me that the targets remained more than 3 miles apart at all times. This has been a recurring problem with IAD Tower and HEF arrivals. They; the IAD Tower; accept the point out on the arriving aircraft and then continue to release departures on conflicting headings. I'm not sure if the Tower feel they're not responsible for making any changes to their operation on receiving this point out; or if they simply don't realize that the headings they issue conflict with the arrival traffic. Recommendation; the easiest fix to this problem is for Local 3 to restrict their headings to no further south than 300 once an HEF arrival; using the 16L ILS and pointed out to them; approaches within 10 miles of IAD. This is less than optimal from an expeditious point of view; but will ensure separation between IAD departures and HEF arrivals.