C206 PLT ENTERS IFR CONDITIONS IN MOUNTAINOUS TERRAIN DURING A VFR FLT.

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I FLY C206 ACFT FOR A LAW ENFORCEMENT AGENCY. MY SUPVR ADVISED ME TO PICK UP 2 LAW ENFORCEMENT OFFICIALS AT TETERBORO ARPT AND TRANSPORT THEM TO BINGHAMTON; NY; AS SOON AS POSSIBLE. I IMMEDIATELY CALLED FLT SVC FOR A BRIEFING AND THEN TRAVELED TO THE ARPT WHERE I RECHKED THE WX ON THE WSI COMPUTER. THE WX BTWN TETERBORO AND BINGHAMTON WAS MARGINAL VFR WITH 2000 FT CEILINGS. THERE WAS AN AIRMET FOR MOUNTAIN OBSCURATION TO THE S OF THIS RTE AND WAS EXPECTED TO BE ENDING BY XA00 PM LCL TIME. IN THE INTEREST OF SAVING TIME; I DECIDED TO GO VFR VIA GPS DIRECT AT 2500 FT MSL. WHILE CRUISING TO THE E OF WILKES-BARRE; I NOTICED THE CEILING WAS GRADUALLY DROPPING AND I QUERIED APCH AND THEY INDICATED A 1900 FT CEILING WHICH IS ABOUT 2900 FT MSL. I THEN PICKED UP BINGHAMTON'S ATIS AND THEY INDICATED A 2200 FT CEILING (3800 FT MSL). I CONTINUED ON COURSE WITH MINOR DEVS AROUND CLOUD BASES. AS I CONTINUED; LARGER AND LARGER DEVS WERE NECESSARY TO MAINTAIN OBSTRUCTION CLRNC WITH RISING TERRAIN AND AN UNFORECAST LOWERING CLOUD DECK. MSA IN THIS SECTOR WAS 3000 FT MSL. UNFORTUNATELY; I CONTINUED ON MY RTE THINKING I WOULD BE ABLE TO DEVIATE AROUND CLOUD BASES BASED ON RPTED CEILINGS AT WILKES-BARRE AND BINGHAMTON AS WELL AS THE FORECAST THAT THE MOUNTAIN OBSCURATION WAS SUPPOSED TO BE WELL TO THE S AND HAVE ENDED OVER 2 HRS EARLIER. FINALLY; I MADE THE DECISION TO MAKE AN EMER CLB TO 3000 FT MSL AS I WAS NO LONGER ABLE TO DEVIATE AROUND CLOUD BASES. I RPTED THE CLB TO BINGHAMTON APCH AND ADVISED THAT I WAS IMC. THE CTLR CLRED ME TO MINIMUM VECTORING ALT OF 3600 FT. AT THIS TIME; I WAS VFR ABOVE A BROKEN LAYER AND BELOW AN OVCST LAYER. I PICKED UP AN IFR CLRNC TO BINGHAMTON ALTHOUGH I WAS ABLE TO MAINTAIN VFR FOR THE REMAINING 25-30 MI OF THE FLT. I FEEL I MADE SOME VERY POOR JUDGEMENTS BASED ON THE PERCEIVED PRESSURE TO GET TO BINGHAMTON AS SOON AS POSSIBLE. IT WOULD HAVE BEEN A MUCH SAFER FLT HAD I FILED AN IFR FLT PLAN OR AT LEAST MADE AN EARLIER DECISION TO DIVERT. I ALSO THINK I CONTINUED BECAUSE I JUST DIDN'T WANT TO ACCEPT THAT THE WX WAS WORSE THAN FORECAST. THERE ARE NO RPTING STATIONS BTWN WILKES-BARRE AND BINGHAMTON TO GET ACCURATE CEILINGS OVER THIS HIGH TERRAIN SO IT WOULD HAVE BEEN SAFER TO DIVERT THAN CONTINUE INTO THE UNKNOWN. IT WAS STUPID AND UNSAFE TO DO WHAT I DID. AS I FOUND OUT THE HARD WAY; WX IN MOUNTAINOUS TERRAIN CAN BE VERY DIFFERENT FROM NEARBY ARPTS LOCATED IN VALLEYS. I FEEL I VERY NARROWLY ESCAPED A TRAGIC CFIT ACCIDENT BECAUSE I WAS BELOW THE MAX ELEVATION FIGURE AT THE TIME I WENT IMC. I LEARNED THAT IT IS BETTER TO DIVERT EARLY THAN TO PRESS ON IN DETERIORATING CONDITIONS HOPING FOR A POSITIVE OUTCOME. UPON LNDG AT BINGHAMTON; I IMMEDIATELY CHKED THE WX AND THERE WAS AN UPDATED AIRMET FOR MOUNTAIN OBSCURATION RIGHT ALONG THE RTE I HAD JUST FLOWN. I FILED AN IFR FLT PLAN DEPARTING BINGHAMTON AND I REALLY FEEL NO ONE SHOULD ATTEMPT MARGINAL VFR FLT IN MOUNTAINOUS TERRAIN AS I HAD TRIED WITH A NEAR DISASTROUS ENDING.