PHL Air Carrier departure experienced TCAS RA with a glider; traffic was issued by ATC; the reporter indicating a directed turn from ATC may have prevented the TCAS RA.

Narrative

I was operating a flight from PHL as pilot not flying [and] the weather was VMC. Approximately 25 NM south of FJC VOR on the 180 radial; level at 4;000 FT; our TCAS issued a TA approaching 3 NM directly ahead of us at our altitude. Within seconds; PHL Departure issued us a Traffic Advisory as well. I responded with 'looking for traffic; how about a turn'. We were given no response. The First Officer and I could not see the traffic and I asked for a turn once again. Again; we were given no response. The next moment the TCAS gave us a RA to descend at 2;000 FT per minute. The First Officer responded appropriately and swiftly; disconnecting the autopilot and simultaneously reduced power and descended at 2;000 FT per minute. I notified PHL Departure that we were responding to a RA; descending. ATC replied by stating 'ROGER'. At about 3;800 FT I was able to get the traffic in sight while it was turning in the direction we were previously occupying. We continued to follow the RA until we were 'clear of conflict'. At that point we were at 3;380 FT. I then told PHL Departure that we were climbing back to our assigned altitude of 4;000 FT. We were then given a frequency change to Allentown Approach upon leveling off at 4;000 FT. The aircraft we were in conflict with was a glider with white over red markings. We were able to see the aircraft when the pilot(s) made a west bound turn into our direction of flight. TCAS played a significant role in avoiding the aircraft. Perhaps if we were issued a turn by ATC; we may not have had to respond to the RA.