DHC8 FLT CREW OPERATING UNDER VFR ENTERS CLASS B AIRSPACE WITHOUT CLRNC AND HAS A TCAS RA. CTLR QUERIES FLT CREW AND GIVES ADVISORY REGARDING THE INFRACTION.

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THE CAPT WANTED TO OPERATE UNDER VFR IN ORDER TO BE ABLE TO FLY DIRECT TO OUR DEST AND; THEREFORE; REDUCE OUR TOTAL FLT TIME. THE FORECAST FOR WHITE PLAINS WAS FOR SCATTERED CLOUDS AT 8000 FT; MUCH LIKE THE CONDITIONS IN PHILADELPHIA. I HAD ONLY FLOWN A PART 121 FLT UNDER VFR ONCE BEFORE; BUT I TRUSTED THAT THE CAPT KNEW WHAT HE WAS DOING. WHEN WE RECEIVED THE ATIS FOR HPN; WE LEARNED THAT THE CLOUD LAYER HAD BECOME BROKEN RATHER THAN SCATTERED. THE CAPT REQUESTED FOR US TO BECOME AN IFR FLT WITH NEW YORK APCH AFTER THEY ASSIGNED US A VECTOR FOR DSCNT; BUT THEY INSTRUCTED US TO REMAIN VFR AND CONTINUE NAVING ON OUR OWN. AT THIS TIME THE CAPT FOUND A HOLE IN THE CLOUD LAYER AND DSNDED US DOWN BELOW THE CLOUDS FROM OUR CRUISING ALT OF 11500 FT TO 7500 FT. IN MY ESTIMATION; WE WERE JUST 500 FT BELOW THE BROKEN CLOUD LAYER. THE DIRECT PATH TO WHITE PLAINS THE CAPT HAD US NAVING ON WAS DIRECTLY THROUGH LGA'S DEP CORRIDOR. WE WERE; THEREFORE; RECEIVING MANY TA'S FROM NEW YORK APCH; AS WELL AS TA'S ON THE TCAS. HOWEVER; WE WERE UNABLE TO ACQUIRE VISUAL CONTACT WITH MOST OF THE TFC BECAUSE OF OUR PROX TO THE CLOUD LAYER. THIS MADE ME FEEL EXTREMELY UNCOMFORTABLE. I LOOKED DOWN AT THE RNAV AND REALIZED THAT WE WERE ONLY 12 MI OUT FROM HPN. AT THIS POINT; I WENT OFF OF THE #1 COM RADIO TO MAKE THE IN-RANGE CALL TO THE COMPANY AND MAKE A PA FOR THE PAX AND FLT ATTENDANT. WHILE I WAS ON THE #2 RADIO; WE RECEIVED AN RA FOR TFC 500 FT ABOVE US; SHOWING THAT WE NEEDED TO DSND AT ABOUT 2500 FPM. THE CAPT WAS NOT USING A HEADSET; AND HIS PUSH-TO-TALK SWITCH HAD INADVERTENTLY BECOME SET ON THE INTERCOM SIDE; SO HE WAS NOT ABLE TO HEAR NEW YORK APCH OVER HIS INTERCOM. THEY HAD GIVEN A TA THAT THE TARGET WAS 1000 FT ABOVE US. HOWEVER; THE CAPT COULD NOT SEE THE TFC AND DID NOT HEAR THE ADVISORY; SO HE COMPLIED WITH THE RA. THIS RESULTED IN US DSNDING TO 6500 FT; THEREFORE; ENTERING THE NEW YORK CLASS B AIRSPACE WITHOUT A CLRNC. NEW YORK HANDED US OVER TO ANOTHER CTLR AND WE WERE CLRED FOR THE VISUAL APCH INTO WHITE PLAINS AND LANDED WITHOUT FURTHER INCIDENT. THE CAPT CLAIMS NOT TO HAVE KNOWN THAT HPN WAS INSIDE THE NEW YORK CLASS B AIRSPACE; AND I DIDN'T CHK BEFORE OUR FLT. I SHOULD HAVE SPENT MORE TIME BECOMING FAMILIAR WITH THE BOUNDARIES OF THE CLASS B AIRSPACE BEFORE THE FLT BEGAN; RATHER THAN TRUSTING THAT THE CAPT WOULD KNOW EVERY ASPECT OF OUR RTE. IN MY OPINION; WE SHOULD HAVE REFUSED TO DSND BELOW THE BROKEN LAYER OF CLOUDS WITHOUT FIRST RECEIVING AN IFR CLRNC; AND I NEEDED TO BE MORE ASSERTIVE IN MAKING MY OPINION KNOWN TO THE CAPT. SUPPLEMENTAL INFO FROM ACN 691455: DURING VFR FLT AT 7500 FT ENRTE TO HPN AND RECEIVING RADAR ADVISORIES FROM NEW YORK APCH; OUR FLT RECEIVED AN RA ON THE TCAS SYS COMMANDING A DSCNT AWAY FROM TFC. WE INITIATED A DSCNT TO COMPLY WITH THE RA INSTRUCTION AND I RPTED THE ALTDEV TO ATC. THE FREQ WAS QUITE CONGESTED. THE ATC CTLR QUERIED US AS TO OUR ACTION. HE SAID THAT HE HAD ADVISED US OF THE TFC AHEAD OF US LEVEL AT 8000 FT. WE DID NOT HEAR THE ADVISORY THAT HE HAD ISSUED AND DID NOT ACQUIRE THE TFC VISUALLY -- OUR RESPONSE TO THE RA WAS MANDATORY.