Flight Crew Commercial
Conflict NMAC
Flight Crew Took Evasive Action
While on arrival; cleared to descend from 8000 FT to 7000 FT and slow to 210 KTS. Observed on TCAS the aircraft 10 miles ahead of us encounter traffic climbing to their altitude. A resolution advisory was declared by Aircraft Z with subsequent change in TCAS readouts by both aircraft. Transmissions by Aircraft Z asking clarification of positive control of the intruding Aircraft Y led me to believe it was a VFR aircraft attempting to fly very near the base of the clouds which we were flying in. Subsequently the intruder Aircraft Y (which had descended to 6000 FT after its near encounter with Aircraft Z); was seen approaching our flight on TCAS. At approximately 5 miles and closing; the intruder Aircraft Y display showed a climb from 6000 FT which would place Aircraft Y in the clouds which we were then flying through. FO queried ATC and requested a turn which ATC granted; a right turn to 090. I turned the heading knob to 090 and pulled; starting the right turn and immediately recognized a rapidly devolving situation. Pressing the autopilot instinctive disconnect button I steepened the bank to 45 degrees and began a climb while commanding 'flight directors off.' At about the same time a RA was alerted by TCAS; followed by the command 'Monitor vertical speed.' We climbed at approximately 2000-3000 FT per min for about 15-20 seconds before the TCAS reported 'Clear of conflict.'; at which time we leveled off (about 8000 FT); and notified ATC of our ability to resume the arrival. No further incidents occurred. The closest distance I saw displayed was 0 miles and 300 ft. Recognizing the possibility that the intruder  Aircraft Y did not have TCAS or would not respond properly to a TCAS event; I chose to continue the turn while climbing; in order to gain 'angles off' the intruder's heading. I believe this to have been the proper course of action. FO made telephone contact with the ATC controller on duty at the time of the incident and was told the Aircraft Y was flying VFR and the proper authorities had been notified to meet him upon arrival. A VFR pilot was attempting to penetrate IMC conditions in order to gain 'VFR on top'; without proper communications and subsequent authorization. In doing so he unwittingly flew his aircraft opposite direction to a major arrival route into one of the busiest airports in the nation. All VFR pilots flying in the vicinity of major airports and their arrival/departure corridors need to be made aware of the risks involved regarding separation of general aviation and commercial/business aircraft. All too often; a VFR pilot wanders into the path of a large aircraft and the events mentioned here happen all over again.
A320 crew experienced an NMAC while on initial descent for approach and landing.  Crew experienced an RA after receiving evasive heading change from ATC. After clear of conflict crew continued the approach and landing.